RFR: Make sure different Verifier levels work

[Roman Kennke]

Am 11.09.2017 um 13:00 schrieb Aleksey Shipilev:
> Setting -XX:ShenandoahVerifyLevel=0 currently fails with thread parity errors. This happens because
> we always instantiate ShenandoahRootsProcessor, and on some paths we don't claim threads, leading to
> this error.
> 
> Fix:
>   http://cr.openjdk.java.net/~shade/shenandoah/verifier-levels-test/webrev.01/
> 
> Testing: hotspot_gc_shenandoah
> 
> Thanks,
> -Aleksey
> 
Yup