selftests/powerpc: Give the bad_accesses test longer to run

On older systems this test takes longer to run (duh), give it five
minutes which is long enough on a G5 970FX @ 1.6GHz.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200819015727.1977134-2-mpe@ellerman.id.au

tools/testing/selftests/powerpc/mm/bad_accesses.c

@@ -139,5 +139,6 @@ static int test(void)
 
 int main(void)
 {
+	test_harness_set_timeout(300);
 	return test_harness(test, "bad_accesses");
 }