some gc tests use 'test.java.opts' and not 'test.vm.opts'

a dozen of gc tests use 'test.java.opts' to get external vm flags, yet actually external flags are split b/w 'test.java.opts' and 'test.vm.opts' and in the majority of cases all external flags are set listed in 'test.vm.opts'. so the tests should be updated to read both.