Made LiveServerTestCase to restore state on exit.

The piece of state is DB connections' allow_thread_sharing attribute
which gets munged test are run when in-memory SQLite databases.

Thanks Anssi for suggesting the possible root cause and Julien for
implementing the fix.

django/test/testcases.py

@@ -1138,4 +1138,11 @@ def tearDownClass(cls):
         if hasattr(cls, 'server_thread'):
             # Terminate the live server's thread
             cls.server_thread.join()
+
+        # Restore sqlite connections' non-sharability
+        for conn in connections.all():
+            if (conn.settings_dict['ENGINE'] == 'django.db.backends.sqlite3'
+                and conn.settings_dict['NAME'] == ':memory:'):
+                conn.allow_thread_sharing = False
+
         super(LiveServerTestCase, cls).tearDownClass()