selftests/powerpc: Harden test for execute-disabled pkeys

Commit 192b6a78

 ("powerpc/book3s64/pkeys: Fix
pkey_access_permitted() for execute disable pkey") fixed a
bug that caused repetitive faults for pkeys with no execute
rights alongside some combination of read and write rights.

This removes the last two cases of the test, which check
the behaviour of pkeys with read, write but no execute
rights and all the rights, in favour of checking all the
possible combinations of read, write and execute rights
to be able to detect bugs like the one mentioned above.

Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/db467500f8af47727bba6b35796e8974a78b71e5.1595821792.git.sandipan@linux.ibm.com