CircleCI: Collect JUnit test reports

This makes it easier to quickly identify which tests failed in a build.
Added 'Debugging' link to TOC in testing doc
Merge pull request #279 from spotify/rohan/fix-tests

Use in-process ZooKeeper instance for testing
Merge pull request #276 from spotify/rohan/test-reports

CircleCI: Collect JUnit test reports
Use in-process ZooKeeper clusters for testing

Use curator's TestingCluster to run in-process ZooKeeper instances for
cluster tests. This is similar to our use of the in-process TestingServer
for tests that require just a single ZooKeeper instance. The difference is
that this is a whole in-process cluster.

This is mostly needed for tests that deal with cluster strategies and
failover.