Themes: Use original template paths when switching blogs.

This fixes a bug introduced by [57129] and [56635] in which deprecating the previous  `TEMPLATEPATH` and `STYLESHEETPATH` constants in favor of `get_template_directory()` and `get_stylesheet_directory()` functions caused the active theme template path to change when using `switch_to_blog()`.

This introduces a new function, `wp_set_template_globals()`, which is called during the bootstrap process to store the template paths to new globals values `$wp_template_path` and `$wp_stylesheet_path`. This restores behavior to how things worked prior to [56635] but retains the ability for template values to be reset for better testability.

Related #18298, #60025.

Props joemcgill, flixos90, mukesh27, swissspidy, manfcarlo, metropolis_john, jeremyfelt.
Fixes #60290.


git-svn-id: https://develop.svn.wordpress.org/trunk@57685 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/includes/abstract-testcase.php

@@ -190,6 +190,10 @@ abstract class WP_UnitTestCase_Base extends PHPUnit_Adapter_TestCase {
 		 */
 		$GLOBALS['wp_sitemaps'] = null;
 
+		// Reset template globals.
+		$GLOBALS['wp_stylesheet_path'] = null;
+		$GLOBALS['wp_template_path']   = null;
+
 		$this->unregister_all_meta_keys();
 		remove_theme_support( 'html5' );
 		remove_filter( 'query', array( $this, '_create_temporary_tables' ) );