Tests: Use a minimal theme for tests.

This functionality was originally added in [38858], using `symlink()` to put a link to the theme in WordPress' `themes` directory. Unfortunately, not all installs have write access to the `themes` directory, causing unit tests to fail.

The new method is to add the test theme directory to `$wp_theme_directories`, and fix the handful of tests that don't expect `$wp_theme_directories` to have multiple entries.

The test install/bootstrap routines now also check that `WP_DEFAULT_THEME` is defined, in case the tests are being run on a system that hasn't upgraded its' `wp-tests-config.php`.

See #31550.
Fixes #38457.


Built from https://develop.svn.wordpress.org/trunk@38907


git-svn-id: http://core.svn.wordpress.org/trunk@38850 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.7-alpha-38906';
+$wp_version = '4.7-alpha-38907';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.