Add -verbose:gc option for test runs

Motivation:

While investigating the recent CI machine crashes, I observed that the
JVM processes spawned by surefire sometimes take up to 1 GiB RAM.
Consuming large amount of memory isn't really a problem, but we need to
make sure no GC trashing is occuring during the tests.

Modifications:

Add -verbose:gc option to the test JVM arguments

Result:

We can determine if there is any GC anomalies going on in our CI
machine.