Themes: Account for uppercase chars when managing themes.

Fixes a bug where the UI wasn't updated after deleting/updating a theme.

Also introduces unit tests for theme management ajax handlers. For now they're
focused on `wp_ajax_update_theme()` but they can include tests for other
handlers as well.

Props chrisjean for initial patch.
Fixes #37924.
 


git-svn-id: https://develop.svn.wordpress.org/trunk@38710 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/data/themedir1/camelCase/index.php

@@ -0,0 +1,5 @@
+<?php
+
+// dummy theme
+
+echo dirname(__FILE__).'/'.basename(__FILE__);

tests/phpunit/data/themedir1/camelCase/style.css

@@ -0,0 +1,11 @@
+/*
+Theme Name: camelCase
+Theme URI: http://example.org/
+Description: This theme has a camelCase slug.
+Version: 1.0
+Author: Minnie Bannister
+Author URI: http://example.com/
+
+This is just a stub to test the loading of the above metadata.
+
+*/