tests: coredump: Change additional boards to directly call k_panic()

The riscv_virtual, hifive_unleashed, and hifive_unmatched boards all
timeout on this test if they actually perform the null dereference when
they are simulated. Whether the null dereference happens seems to depend
on the compiler used as well as whether optimizations are enabled though.

To make this more consistent, handle these boards in the same way as others
which use Renode simulation and have them directly call k_panic().

Signed-off-by: Jonathon Penix <jpenix@quicinc.com>