a few compiler/debug tests don't check exit code

compiler/debug/TestGenerateStressSeed.java, TestStressCM.java, and TestStressIGVNAndCCP.java don't check the exit code of spawned JVMs, as a result, they might miss misconfiguration and/or defects. in fact, TestStressIGVNAndCCP.java uses a wrong flag name (TraceIterativeIGVN instead of TraceIterativeGVN) and this was unnoticed b/c we didn't check the exit code.