tests: PortalMock: Don't shut down the werkzeug server
tests: PortalMock: Don't shut down the werkzeug server

`flask.request.environ.get('werkzeug.server.shutdown')` now returns None
because that functionality was removed (see
https://github.com/pallets/werkzeug/pull/2276 and
https://github.com/pallets/werkzeug/issues/1752).

This test process is short lived so it doesn't matter if this thread
stays running. `portal_mock` is a per-function fixture, so each test
will get its own PortalMock server with its own thread.

stb-tester/stbt-rig