selftests/powerpc: Skip energy_scale_info test on older firmware

Older machines don't have the firmware feature that enables the code
this test is testing. Skip the test if the sysfs directory doesn't
exist. Also use the FAIL_IF() macro to provide more verbose error
reporting if an error is encountered.

Fixes: 57201d65 ("selftest/powerpc: Add PAPR sysfs attributes sniff test")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220619233103.2666171-1-mpe@ellerman.id.au