Polluted table_prefix and switched_stack in multisite unit tests

[jeremyfelt]

At present, and seemingly since [27511], there are many test failures in multisite around various queries. Through these failures, I was seeing several Cannot use object of type WP_Error as array messages appear because the tests were trying to use database tables that didn't exist due to a polluted $_wp_switched_stack that still thought it was on site 67 after a tear down.

As of [28910], this gets even crazier and pollutes $table_prefix so that we end up with queries for wptests_66_66_66_67_67_67_68_terms, etc...

[jeremyfelt]

28692.diff loops through restore_current_blog() on test tear down until we are back to the base site. It also resets the global $table_prefix when testing multisite's bootstrap, as we include ms-settings.php repeatedly, which relies on this to be pretty clean.

[SergeyBiryukov]

Related: #27704

[jeremyfelt]

28692.2.diff wraps ms_is_switched() with an is_multisite() check so that our regular unit tests aren't completely broken due to a non-existent function.

[wonderboymusic]

In 28943:

Avoid polluted $table_prefix and $_wp_switched_stack in multisite unit tests.
Add WP_TESTS_TABLE_PREFIX in bootstrap.php to ensure the original value for $table_prefix is always available.

Props jeremyfelt.
Fixes #28692.