refactor: Tests cleanup and better test isolation

[JuroOravec]

• Removed the use of dedent() in test assertions that used assertHTMLEqual
• Components are now registered inside tests (e.g. in setUpClass method), and the component registry is cleared between tests.
• Turned out that autodiscovery is also a stateful action, because the code in the imported python files is executed only once - in other words, if the file/module as already been loaded, then the code will not be run. For this I added a helper to clean up after autodiscovery, so it can be run in multiple tests. See https://github.com/EmilStenstrom/django-components/pull/452/files#diff-9d24dc3dc9330ef153577f547b4024b50a095ce91290796e44b7a59dcd7b9119R65
  • Note: I also updated the autodiscover function to accept an optional mapping function for the import paths. This was needed because in tests, the components dir is not at the top-level, but under tests/.
• There was also one duplicate test that I removed

Closes https://github.com/EmilStenstrom/django-components/issues/436