Add WordPress timing tests

[schlessera]

Feature description

We don't have an obvious way right now to test anything that happens before the 'wp' hook, as we're using the WP_UnitTestCase. This leads to issues like the https://github.com/ampproject/amp-wp/issues/4525 because we can never catch any timing problems that can happen before the 'wp' action.

It might be worthwhile to set up a separate set of tests that use SHORT_INIT or some other hack to achieve the same thing) to be able to test our plugin against WordPress, but control which actions happen and test for earlier conflicts.

This would also allow us to add regression tests to fixes like the one for the issue above, where we can only test manually right now.

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Acceptance criteria

• Tests can run at an earlier action than 'wp'.
• Tests still load the AMP plugin and any other additional extensions we need as required.

Implementation brief

• Create a new TestCase implementation that loads WordPress via SHORT_INIT.
• Add a mechanism to let the test choose at which action it wants to run, like a go_to_action() method.

QA testing instructions

Demo

Changelog entry

[westonruter]

We could also just empty out the $wp_actions array.

[schlessera]

That won't have the same effect, as the actions will already have run and modified all the globals. If we want to catch timing issues, we have to run the tests with the real setup. otherwise we're testing something irrelevant and meaningless.