Tests: Use more appropriate assertions in `get_themes()` tests.

This replaces instances of `assertTrue( is_file( ... ) )` followed by `assertTrue( is_readable( ... ) )` with `assertFileIsReadable()` to use native PHPUnit functionality.

The `assertFileIsReadable()` method was introduced in PHPUnit 5.6. As the minimum supported PHPUnit version has been raised to PHPUnit 5.7.21, it can now be used.

Follow-up to [51543], [51574].

Props jrf.
See #53363.
Built from https://develop.svn.wordpress.org/trunk@51579


git-svn-id: http://core.svn.wordpress.org/trunk@51190 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51578';
+$wp_version = '5.9-alpha-51579';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.