Inject IHTTPClientService

Otherwise the unit test execution will do a ton of external HTTP requests which fail and then timeout…

See https://blackfire.io/profiles/compare/3c67acfa-a11e-4aec-bcd4-c945b006f01e/graph for reference

Pretty similar to https://github.com/nextcloud/server/pull/1565



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