Thumbnails: add None check on lib_name

`find_library` could return None, which would not cause an OSError
from `loadLibrary`, making the plugin (falsely) think the library is available

Also fixed  wrong method call to skip test in that case

See #1277

beetsplug/thumbnails.py

@@ -241,6 +241,8 @@ class GioURI(URIGetter):
     def get_library(self):
         lib_name = ctypes.util.find_library("gio-2")
         try:
+            if not lib_name:
+                return False
             return ctypes.cdll.LoadLibrary(lib_name)
         except OSError:
             return False

test/test_thumbnails.py

@@ -265,7 +265,7 @@ class ThumbnailsTest(unittest.TestCase, TestHelper):
         gio = GioURI()
         plib = PathlibURI()
         if not gio.available:
-            self.skip("GIO library not found")
+            self.skipTest("GIO library not found")
 
         self.assertEqual(gio.uri("/foo"), b"file:///")  # silent fail
         self.assertEqual(gio.uri(b"/foo"), b"file:///foo")