Basic WACZ tests

[rebeccacremona]

See ENG-967.

This PR adds a few basic tests of Perma's new WACZ-related features:

• one test for each branch of the "should we launch a WARC or a WACZ playback" code
• one functional test that actually does a WACZ playback, and confirms that it works.

To make the functional test easier to write:

• I set WACZ_SAVING_ENABLED = True in testing (seems harmless, right?)
• I did some light refactoring of the functional test fixtures to convert the logged_in_user fixture out into a utility function, for clarity and increased flexibility
• I created a new user fixture for whom the Django waffle "wacz-playback" flag is set to True.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 61.11111% with 7 lines in your changes missing coverage. Please review.

Project coverage is 68.94%. Comparing base (5cc0efc) to head (5eea1a2). Report is 7 commits behind head on develop.

Files	Patch %	Lines
perma_web/conftest.py	61.11%	7 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #3551 +/- ## =========================================== + Coverage 68.87% 68.94% +0.06% =========================================== Files 48 48 Lines 6856 6900 +44 =========================================== + Hits 4722 4757 +35 - Misses 2134 2143 +9 ```

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