Make TestAcidGuarantees usable for system testing.

Currently, the TestAcidGuarantees run via main() will always abort with an NPE because it digs into a non-existant HBaseTestingUtility for a flusher thread. We should tool this up so that it works properly from the command line. This would be a very useful long running test when used in conjunction with fault injections to verify row acid properties.