test_router: factor common test code

Common test code is factored in test case methods.
test_router: remove duplicate test
Flush Redis cache prior to each unit test

This allows us to run unit tests in any order, or individually.
Fix insecure public/secret key verification

If a correct project_id was passed to the backend_request with an
invalid public/secret key pair, the request was forwarded nonetheless to
sentry.
Merge pull request #10 from Polyconseil/fix-key-check

Fix insecure public/secret key verification