Add a class FlagGuard that allows for easier modification of flags for unit tests. A TEST_VM unit tests that wants to change a flag simply has to create a FlagGuard:

FlagGuard g("MaxGCPauseMillis");

together with a simple macro, GUARD_FLAG, a test only has to write:

GUARD_FLAG(MaxGCPauseMillis);

and can then the test can change the flag however it wants, the flags value will be restored when the test is done.