tests: Migrate to pytest and cleanup

Migrate all swh-web unit tests from unittest to pure pytest which
implies the following:

• all tests are turned from a class method to a simple function
• replace previous assertions from unittest.TestCase to simple calls to Python assert
• previous features offered by the former TestCase classes are turned into pytest fixtures
• use pytest-mock fixture instead of decorator from unittest.patch
• use requests_mock_datadir fixture from swh-core simplifying the setup of mocked HTTP requests
• expose some Django assertions to be used with pytest

Some cleanups have also been performed on the tests implementation, notably:

• Better formatting of string litterals
• Overall code style improvements
• Removal of tests related to origin ids

Closes T2062