Disable tests for JDK-8280481 on Graal

The test for JDK-8280481 ("Duplicated stubs to interpreter for static calls") fails when run on Graal:

java.lang.RuntimeException: No static stubs reused: distinct 4, in total 4
at compiler.sharedstubs.SharedStubToInterpTest.checkOutput(SharedStubToInterpTest.java:95)
at compiler.sharedstubs.SharedStubToInterpTest.runTest(SharedStubToInterpTest.java:78)
at compiler.sharedstubs.SharedStubToInterpTest.main(SharedStubToInterpTest.java:84)
at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
at java.base/java.lang.reflect.Method.invoke(Method.java:580)
at com.sun.javatest.regtest.agent.MainActionHelper$AgentVMRunnable.run(MainActionHelper.java:333)
at java.base/java.lang.Thread.run(Thread.java:1570)

The test should exclude Graal until such time that Graal implements the same optimization.