Mock HTTPClientService

These tests were not using any kind of mocks at all! This made the test execution time took 2 minutes instead of now less than 1 second… (because multiple HTTP requests were queued and timed-out then…)

See https://blackfire.io/profiles/compare/6d2611f2-ad08-47b6-973b-2b5f9c8b96d2/graph

 for reference

Signed-off-by: Lukas Reschke <lukas@statuscode.ch>