In gc/shenandoah jtreg tests, there are !vm.graal.enabled clauses in some @requires, but not in the other. Since JDK 15 and JDK-8249000, we don't need the explicit Graal check in those tests.

But, for consistency within the tests and consistency across the JDK releases, we might consider checking for Graal explicitly. This would enable us to transplant the tests from 15u to 11u without remembering to check for Graal additionally.

Or, we can drop all Graal checks from current gc/shenandoah test base, and let 11u backports figure the rest out.