Be sensitive to token in tests

[ManuelBahr]

Currently, the test_token test runs through but actually should fail. The test obviously is not sensitive to the token setting. In practice it works, but the test should represent that behaviour.

[MathMagique]

This is the test in question: https://github.com/blue-yonder/azure-cost-mon/blob/master/tests/test_app.py#L32

[ManuelBahr]

This should be the trick to check for the header https://github.com/getsentry/responses#request-callback