Improve temp dir handling in tests

Specifically, on Windows, TestCaseThatUploadsFiles currently shows
errors on interpreter exit. These are caused by
TemporaryDirectory.cleanup() not being able to delete files because they
are still open in python.exe. Now at least, the errors appear during
tearDown().

winget/tests/util.py

@@ -9,15 +9,19 @@ from tenants.models import Tenant
 When a test case uploads files, they by default get placed into media/. This
 folder (out of the box) is not cleaned up after the test case is run.
 TestCaseThatUploadsFiles fixes this. It makes sure that files are uploaded into
-a temporary directory. This directory is cleaned up when the Python interpreter
-exits. This is achieved by keeping a global reference to the TemporaryDirectory
-instance. Once this instance goes out of scope, Python guarantees that the
-temporary directory (and its contents) are removed.
+a temporary directory. This directory is cleaned up at the end of each test.
 """
-_MEDIA_ROOT = TemporaryDirectory()
-@override_settings(MEDIA_ROOT=_MEDIA_ROOT.name)
 class TestCaseThatUploadsFiles(TestCase):
-	pass
+	def setUp(self):
+		super().setUp()
+		self.media_root = TemporaryDirectory()
+		media_root_path = self.media_root.name
+		self.settings_override = override_settings(MEDIA_ROOT=media_root_path)
+		self.settings_override.enable()
+	def tearDown(self):
+		super().tearDown()
+		self.settings_override.disable()
+		self.media_root.cleanup()
 
 class MultiUploadInMemoryFile(SimpleUploadedFile):
 	"""