RFR(T): 8223427: [TESTBUG] Disable JTReg Shenandoah tests when Graal is enabled

[Pengfei Li (Arm Technology China)]

Hi Aleksey,

> Seeing that it is trivial and only affects Shenandoah tests, I'd say one review is
> enough.
> 
> Do you need a sponsor to push?

Yes, I would be happy if you could help to push.

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Thanks,
Pengfei