Just dropping these tests would basically mean zero coverage for verifyAssertion, since its stubbed out in the current tests.  Other alternatives that jump out at me include 

# pulling the verifier out into it's own object and moving these tests there
# completely re-writing the #signin tests to not mock the verifier (maybe using Sinon's mock XHR stuff?)

I'm not helping the perfect be the enemy of the good here, or in having this card blow up.  So my suggestion would be to pick the solution you like from above (my gut is that pulling out the verifier is lighter weight), and add an XXX comment suggestion doing that at some point in the future.