selftests/powerpc: Fix ptrace-pkey for default execute permission change

The test case assumes execute-permissions of unallocated keys are
enabled by default, which is incorrect.

Reviewed-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Signed-off-by: Ram Pai <linuxram@us.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>