[test] prevent timeouts in serviceability/tmtools/jstat/{GcTest02,GcCauseTest02}.java

The two tests hotspot/jtreg/serviceability/tmtools/jstat/{GcTest02,GcCauseTest02}.java produce more than 90_000 classes until they eat up ~70% of the 128M meta space they run with. The loading of each of these classes triggers a full dependency check for ALL nmethods in debug/fastdebug builds because 'VerifyDependencies' is 'true' there. This slows down the tests from about 3 sec. in the opt build to about 88 sec. in the fastdebug build on x86_64 and from about 4 sec. to about 560 sec. on ppc64.

Because the tests are not about dependency checking, it makes sense to switch of 'VerifyDependencies' if they are run inside a debug/fastdebug VM and decrease the execution time down to about 6 sec. on both x86_64 and ppc64.