hs_service/helper_functions test seems unreliable on slow systems

Hi! I've seen this test failure a few times on Travis:

hs_service/helper_functions: [forking] 
  FAIL src/test/test_hs_service.c:837: assert(ip->time_to_expire OP_GE now + INTRO_POINT_LIFETIME_MIN_SECONDS): 1537602275 vs 1537602297
  [helper_functions FAILED]

It seems to happen intermittently, so I think it might be one of those things where a slow-running machine fails the test because of the unexpected passage of time.

Is it possible to mock time here?

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information