jfr/event/oldobject/TestClassLoaderLeak.java fails when GC cycles are not happening

This reliably fails with Serial (ignored by @requires), Shenandoah and Z:

$ CONF=linux-x86_64-server-fastdebug make run-test TEST=jdk/jfr/event/oldobject/TestClassLoaderLeak.java TEST_VM_OPTS="-XX:+UseShenandoahGC"

...

STDERR:
java.lang.RuntimeException: Could not find class leak
at jdk.test.lib.Asserts.fail(Asserts.java:594)
at jdk.jfr.event.oldobject.TestClassLoaderLeak.main(TestClassLoaderLeak.java:80)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:568)
at com.sun.javatest.regtest.agent.MainWrapper$MainThread.run(MainWrapper.java:127)
at java.base/java.lang.Thread.run(Thread.java:833)


I believe this test implicitly wants at least one GC cycle to happen so that ObjectSampler::oop_storage_gc_notification turns the sampled objects "old". An easy way to do this is to trim the Java heap size. If that does not work, we would need to go to MXBeans and watch GC cycles happening.