Fix unrestored AutotagStub on two TestCases

* Call AutotagStub.restore() during the tearDown of
test_mbsubmit.MBSubmitPluginTest and test_plugins.PromptChoicesTest,
which could potentially lead to other tests calling mocked versions
of autotag.mb.match_album, .match_track, .album_for_id and .track_for_id
instead of the real functions.

test/test_mbsubmit.py

@@ -34,6 +34,7 @@ class MBSubmitPluginTest(TerminalImportSessionSetup, unittest.TestCase,
     def tearDown(self):
         self.unload_plugins()
         self.teardown_beets()
+        self.matcher.restore()
 
     def test_print_tracks_output(self):
         """Test the output of the "print tracks" choice."""

test/test_plugins.py

@@ -419,6 +419,7 @@ class PromptChoicesTest(TerminalImportSessionSetup, unittest.TestCase,
         self.input_options_patcher.stop()
         self.teardown_plugin_loader()
         self.teardown_beets()
+        self.matcher.restore()
 
     def test_plugin_choices_in_ui_input_options_album(self):
         """Test the presence of plugin choices on the prompt (album)."""