Designates a method to be called only once, just before the start of the
benchmark run. It is always called from global thread 0 for the given driver.
No other thread will be started until this method finishes. Note that only
one method is allowed for the OnceBefore designation. If Faban finds more
than one method with the OnceBefore annotation, it will throw a
DefinitionException at startup.
The OnceBefore and OnceAfter annotations are used mainly for auditing
the state of the SUT before and after the benchmark run. A CustomMetrics
shall be used to report the audit results.