Miscellaneous test suite improvements

[johnbillion]

This is a tracking ticket for miscellaneous improvements to the test suite that don't warrant individual tickets.

[johnbillion]

In 39172:

Themes: Re-use fixtures in the wp_get_document_title() tests.

See #38716

[johnbillion]

In 39173:

Build/Test Tools: Utilise assertFileExists() and assertFileNotExists() in more places.

See #38716

[johnbillion]

In 39174:

Build/Test Tools: Utilise assertWPError() and assertNotWPError() in more places.

See #38716

[johnbillion]

In 39189:

Build/Test Tools: Re-use a bunch of fixtures in test classes for user and XMLRPC tests.

Shaves a couple of seconds off of the tests.

See #30017, #38716

[johnbillion]

In 39364:

Build/Test Tools: Add an extra WP_Error assertion when testing a valid user activation key. This provides a better failure message if the assertion does fail.

See #38716

[johnbillion]

In 39554:

Build/Test Tools: Remove commented out tests that have existed in an unimplemented state since the dawn of the test infrastructure.

See #38716

[johnbillion]

In 39555:

Build/Test Tools: Reuse another fixture in the user capability tests.

See #38716