selftests/powerpc: Return errors from all tests

Fix some tests which weren't returning an error code from main.

Although these tests only ever return success, they can still fail if
they time out and the harness kills them. If that happens they still
return success to the shell, which is incorrect and confuses the higher
level error reporting.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://patch.msgid.link/20241106130453.1741013-4-mpe@ellerman.id.au