Use blocking reader to fix race in test

The TestCanceledContext test in twofactorverify_test.go tests the scenario wherein the request context gets canceled in the middle of the request.

A race can occur though when the input gets read before the context is canceled which can result to a different error: instead of having the context canceled, the OTP will be blank.

To fix it, we use a blocking reader as input to simulate "waiting for input" scenario. This way, reading the input will never be finished and the context cancelation can be done appropriately.

Closes #602 (closed)