Fixed #21158 - Set an implicit wait for selenium

Added command to set the implicit wait immediately after creating the
WebDriver instance in AdminSeleniumWebDriverTestCase.
Changed JavascriptI18nTests to inherit from AdminSeleniumWebDriverTestCase
to avoid having to duplicate this logic.
I don't like importing from admin.tests here. Maybe we could factory out the minimum shared logic (which I believe is just setupClass and tearDownClass) and put it somewhere in django.test.