Fix flakiness in BasicCallTests

One test was flaky because the call wasn't making it to the
incallservice in time. Fix this by waiting on the correct future and the
correct handler.

Change-Id: Iedf341a39f8df8e6efe5d5d90c19d4339fd64a5d
Fixes: 120828657
Test: run test 100 times
2 files changed