[1.5.x] Made a couple of selenium tests wait for page loaded

The admin_widgets tests were issuing click() to the browser but
didn't wait for the effects of those clicks. This caused the resulting
request to be processed concurrently with the test case. When using
in-memory SQLite this caused weird failures.

Also added wait_page_loaded() to admin selenium tests for code
reuse.

Fixed #19856, backpatch of 50677b29af

tests/regressiontests/admin_inlines/tests.py

@@ -546,15 +546,7 @@ class SeleniumFirefoxTests(AdminSeleniumWebDriverTestCase):
         self.selenium.find_element_by_name('profile_set-2-last_name').send_keys('2 last name 2')
 
         self.selenium.find_element_by_xpath('//input[@value="Save"]').click()
-
-        try:
-            # Wait for the next page to be loaded.
-            self.wait_loaded_tag('body')
-        except TimeoutException:
-            # IE7 occasionnally returns an error "Internet Explorer cannot
-            # display the webpage" and doesn't load the next page. We just
-            # ignore it.
-            pass
+        self.wait_page_loaded()
 
         # Check that the objects have been created in the database
         self.assertEqual(ProfileCollection.objects.all().count(), 1)