TMF: Fix trace synchronization memory performance test, to reduce variability

TMF: Fix trace synchronization memory performance test, to reduce variability

And document a bit more how to use the USED_JAVA_HEAP metric, so that it gives
accurate results.

Change-Id: Id776ab4cbd70f1f22bcfa9fc5efe95b97b6c4b77
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/30999
Tested-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>