Race condition in IsMasterIsFalseDuringStepdown unittest

There's a race in the test for "SERVER-37065 Report ismaster:false on learning of new term". The test asserts that "ismaster" is false right away when the primary learns of a new term, which is fine, and it also asserts that "secondary" is still false because the stepdown is incomplete. If the primary transitions to secondary quickly enough, the second assert fails.

Fix the race with the "blockHeartbeatStepdown" failpoint.