selftests/powerpc: Give some tests longer to run

Some of these long running tests can time out on heavily loaded
systems, give them longer to run.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

tools/testing/selftests/powerpc/stringloops/memcmp.c

@@ -154,5 +154,6 @@ static int testcases(void)
 
 int main(void)
 {
+	test_harness_set_timeout(300);
 	return test_harness(testcases, "memcmp");
 }