Added the `wait_until()` and `wait_loaded_tag()` methods to...

Added the `wait_until()` and `wait_loaded_tag()` methods to `AdminSeleniumWebDriverTestCase` to prevent some concurrency issues with in-memory SQLite database access in the admin Selenium tests. Thanks to Florian Apolloner, Anssi Kääriäinen and Aymeric Augustin for their help debugging this problem.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17283 bcc190cf-cafb-0310-a4f2-bffc1f526a37