Tests: Restore the database connection earlier when switching test groups.

When plugins don't disable the `backupGlobals` PHPUnit option in their own tests, `$wpdb` is backed up and restored between classes of tests. The serialisation process used for this broke the database connection. This previously wasn't a problem, as it was reconnecting before each test.

[38398] introduced some changes that required the connection to be available in `setUpBeforeClass()`, earlier than in was previously reconnecting. This didn't cause warnings in Core, but it did cause warnings for plugins that don't disable the `backupGlobals` option.

The database connection now reconnects in `setUpBeforeClass()`. This change also fixes a few Core tests that weren't calling `parent::setUpBeforeClass()` or `parent::tearDown()` correctly.

Fixes #39327.


Built from https://develop.svn.wordpress.org/trunk@39626


git-svn-id: https://core.svn.wordpress.org/trunk@39566 1a063a9b-81f0-0310-95a4-ce76da25c4cd