Make use of the new JUnit functionality introduced in the previous
commit to 'Assume' that perfomance- and timing-sensitive tests should
run only when TestGroup.PERFORMANCE is selected, i.e. when
-PtestGroups="performance" has been provided at the Gradle command line.
The net effect is that these tests are now ignored by default, which
will result in far fewer false-negative CI build failures due to
resource contention and other external factors that cause slowdowns.
We will set up a dedicated performance CI build to run these tests on
an isolated machine, etc.
Issue: SPR-9984
60032e0012