sanitycheck: make --subtest and --test mutually exclusive

They're effectively mutually exclusive already because
options.sub_test in main() immediately discards any --test
argument(s). This commit preempts user confusion thanks to this new
message:

 sanitycheck: error: argument --sub-test: not allowed with
    argument -s/--test

Signed-off-by: Marc Herbert <marc.herbert@intel.com>
1 file changed