Fix Python warning during LiveServerPort

[Jon Dufresne]

When running tests with Python warnings enabled, LiveServerPort produces a warning of the form:

Exception in thread Thread-1:
Traceback (most recent call last):
  File "/usr/lib64/python3.7/threading.py", line 917, in _bootstrap_inner
    self.run()
  File "django/test/testcases.py", line 1399, in run
    connections.close_all()
  File "django/db/utils.py", line 224, in close_all
    connection.close()
  File "django/db/backends/sqlite3/base.py", line 244, in close
    self.validate_thread_sharing()
  File "django/db/backends/base/base.py", line 531, in validate_thread_sharing
    % (self.alias, self._thread_ident, _thread.get_ident())
django.db.utils.DatabaseError: DatabaseWrapper objects created in a thread can only be used in that same thread. The object with alias 'default' was created in thread id 139685002331648 and this is thread id 139684747486976.

This occurs because multiple classes set DatabaseWrapper.allow_thread_sharing to false upon test tear down. The base test class and the temporarily created test class. In other words, nesting the setup/teardown of DatabaseWrapper.allow_thread_sharing isn't always handled cleanly.