Add test plans, run tests with NSZombieEnabled

[jberkel]

    Restarting after unexpected exit, crash, or test timeout in CustomAggregationTests.testCustomStringAgg(); summary will include totals from previous launches.

[jberkel]

Test plans allow running tests with different options, we should try to use NSZombie, guard malloc etc to detect memory issues, especially important since the code makes heavy use of the Unsafe* APIs.