_resolve_genre as list tests, add test_to_delimited_string

- Adapt tests to _resolve_genres returning a list with not yet formatted genres.
- Rename and adapt test_count -> test_to_delimited_string. Note that the
  new function does not apply whitelist, prefer anything. It just cuts
  to count and formats!

test/plugins/test_lastgenre.py

@@ -46,46 +46,49 @@ class LastGenrePluginTest(BeetsTestCase):
     def test_default(self):
         """Fetch genres with whitelist and c14n deactivated"""
         self._setup_config()
-        assert self.plugin._resolve_genres(["delta blues"]) == "Delta Blues"
+        assert self.plugin._resolve_genres(["delta blues"]) == ["delta blues"]
 
     def test_c14n_only(self):
         """Default c14n tree funnels up to most common genre except for *wrong*
         genres that stay unchanged.
         """
         self._setup_config(canonical=True, count=99)
-        assert self.plugin._resolve_genres(["delta blues"]) == "Blues"
-        assert self.plugin._resolve_genres(["iota blues"]) == "Iota Blues"
+        assert self.plugin._resolve_genres(["delta blues"]) == ["blues"]
+        assert self.plugin._resolve_genres(["iota blues"]) == ["iota blues"]
 
     def test_whitelist_only(self):
         """Default whitelist rejects *wrong* (non existing) genres."""
         self._setup_config(whitelist=True)
-        assert self.plugin._resolve_genres(["iota blues"]) == ""
+        assert self.plugin._resolve_genres(["iota blues"]) == []
 
     def test_whitelist_c14n(self):
         """Default whitelist and c14n both activated result in all parents
         genres being selected (from specific to common).
         """
         self._setup_config(canonical=True, whitelist=True, count=99)
-        assert (
-            self.plugin._resolve_genres(["delta blues"]) == "Delta Blues, Blues"
-        )
+        assert self.plugin._resolve_genres(["delta blues"]) == [
+            "delta blues",
+            "blues",
+        ]
 
     def test_whitelist_custom(self):
         """Keep only genres that are in the whitelist."""
         self._setup_config(whitelist={"blues", "rock", "jazz"}, count=2)
-        assert self.plugin._resolve_genres(["pop", "blues"]) == "Blues"
+        assert self.plugin._resolve_genres(["pop", "blues"]) == ["blues"]
 
         self._setup_config(canonical="", whitelist={"rock"})
-        assert self.plugin._resolve_genres(["delta blues"]) == ""
+        assert self.plugin._resolve_genres(["delta blues"]) == []
 
-    def test_count(self):
-        """Keep the n first genres, as we expect them to be sorted from more to
-        less popular.
+    def test_to_delimited_string(self):
+        """Keep the n first genres, format them and return a
+        separator-delimited string.
         """
-        self._setup_config(whitelist={"blues", "rock", "jazz"}, count=2)
+        self._setup_config(count=2)
         assert (
-            self.plugin._resolve_genres(["jazz", "pop", "rock", "blues"])
-            == "Jazz, Rock"
+            self.plugin._to_delimited_genre_string(
+                ["jazz", "pop", "rock", "blues"]
+            )
+            == "Jazz, Pop"
         )
 
     def test_count_c14n(self):
@@ -95,31 +98,28 @@ class LastGenrePluginTest(BeetsTestCase):
         )
         # thanks to c14n, 'blues' superseeds 'country blues' and takes the
         # second slot
-        assert (
-            self.plugin._resolve_genres(
-                ["jazz", "pop", "country blues", "rock"]
-            )
-            == "Jazz, Blues"
-        )
+        assert self.plugin._resolve_genres(
+            ["jazz", "pop", "country blues", "rock"]
+        ) == ["jazz", "blues"]
 
     def test_c14n_whitelist(self):
         """Genres first pass through c14n and are then filtered"""
         self._setup_config(canonical=True, whitelist={"rock"})
-        assert self.plugin._resolve_genres(["delta blues"]) == ""
+        assert self.plugin._resolve_genres(["delta blues"]) == []
 
     def test_empty_string_enables_canonical(self):
         """For backwards compatibility, setting the `canonical` option
         to the empty string enables it using the default tree.
         """
         self._setup_config(canonical="", count=99)
-        assert self.plugin._resolve_genres(["delta blues"]) == "Blues"
+        assert self.plugin._resolve_genres(["delta blues"]) == ["blues"]
 
     def test_empty_string_enables_whitelist(self):
         """Again for backwards compatibility, setting the `whitelist`
         option to the empty string enables the default set of genres.
         """
         self._setup_config(whitelist="")
-        assert self.plugin._resolve_genres(["iota blues"]) == ""
+        assert self.plugin._resolve_genres(["iota blues"]) == []
 
     def test_prefer_specific_loads_tree(self):
         """When prefer_specific is enabled but canonical is not the
@@ -131,15 +131,15 @@ class LastGenrePluginTest(BeetsTestCase):
     def test_prefer_specific_without_canonical(self):
         """Prefer_specific works without canonical."""
         self._setup_config(prefer_specific=True, canonical=False, count=4)
-        assert (
-            self.plugin._resolve_genres(["math rock", "post-rock"])
-            == "Post-Rock, Math Rock"
-        )
+        assert self.plugin._resolve_genres(["math rock", "post-rock"]) == [
+            "post-rock",
+            "math rock",
+        ]
 
     def test_no_duplicate(self):
         """Remove duplicated genres."""
         self._setup_config(count=99)
-        assert self.plugin._resolve_genres(["blues", "blues"]) == "Blues"
+        assert self.plugin._resolve_genres(["blues", "blues"]) == ["blues"]
 
     def test_tags_for(self):
         class MockPylastElem: