selftests/powerpc: Detect taint change in mitigation patching test

Currently the mitigation patching test errors out if the kernel is
tainted prior to the test running.

That causes the test to fail unnecessarily if some other test has caused
the kernel to be tainted, or if a proprietary or force module is loaded
for example.

Instead just warn if the kernel is tainted to begin with, and only
report a change in the taint state as an error in the test.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://patch.msgid.link/20241106130453.1741013-5-mpe@ellerman.id.au