Assert during stackwalk

[Roman Kennke]

>> (Also, is this all Loom tests, or should I also run other tests too?
>>
>> make run-test TEST=java/lang/Continuation 
>> TEST_VM_OPTS="-XX:+UnlockDiagnosticVMOptions -XX:+UseShenandoahGC 
>> -XX:ShenandoahGCHeuristics=aggressive"
> 
> TEST=java/lang/Thread/virtual will run a lot more tests, including 
> several stress tests.

Thanks, Alan!

Roman