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.

Merges [39626] to the 4.7 branch.

Fixes #39327.


Built from https://develop.svn.wordpress.org/branches/4.7@39627


git-svn-id: http://core.svn.wordpress.org/branches/4.7@39567 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.7.1-alpha-39615';
+$wp_version = '4.7.1-alpha-39627';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.