powerpc/test_emulate_step: Enhancement to test negative scenarios

add provision to declare test is a negative scenario, verify
whether emulation fails and avoid executing it.

Signed-off-by: Balamuruhan S <bala24@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200626095158.1031507-2-bala24@linux.ibm.com