tests: Don't generate binary output from checks.

The tests will end up logging the check output, and spewing random
binary all over a terminal never results in anything good.
WaitForLeader: Also wait for a non-zero index.

On my laptop, I'm currently seeing a huge number of intermittent test
failures all related to WaitForLeader returning after the test node has
become a leader, but before it has actually finished starting up, in
particular, performing the serf/Raft reconciliation.

Waiting for the index to become nonzero makes the tests pass reliably,
by also blocking until the new leader has started committing state.
Merge pull request #175 from nelhage/fix-tests

Fix some issues in tests.