Tests: Fix Tests_Theme tests to run (and pass) cross-OS.

Uses `DIRECTORY_SEPARATOR` in closures for cross-OS differences.

Follow-up to [56635].

Props jrf.
See #61530.

git-svn-id: https://develop.svn.wordpress.org/trunk@59054 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/theme.php

@@ -976,7 +976,7 @@ public function data_get_stylesheet_directory_with_filter() {
 				'block-theme',
 				'stylesheet_directory',
 				static function ( $dir ) {
-					return str_replace( realpath( DIR_TESTDATA ) . '/themedir1', '/fantasy-dir', $dir );
+					return str_replace( realpath( DIR_TESTDATA ) . DIRECTORY_SEPARATOR . 'themedir1', '/fantasy-dir', $dir );
 				},
 				'/fantasy-dir/block-theme',
 			),
@@ -1036,7 +1036,7 @@ public function data_get_template_directory_with_filter() {
 				'block-theme',
 				'template_directory',
 				static function ( $dir ) {
-					return str_replace( realpath( DIR_TESTDATA ) . '/themedir1', '/fantasy-dir', $dir );
+					return str_replace( realpath( DIR_TESTDATA ) . DIRECTORY_SEPARATOR . 'themedir1', '/fantasy-dir', $dir );
 				},
 				'/fantasy-dir/block-theme',
 			),