selftests: net: ignore timing errors in txtimestamp if KSFT_MACHINE_SLOW

This test is time sensitive. It may fail on virtual machines and for
debug builds.

Similar to commit c41dfb0d ("selftests/net: ignore timing errors in
so_txtime if KSFT_MACHINE_SLOW"), optionally suppress failure for timing
errors (only).
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/net/txtimestamp.c

@@ -163,7 +163,8 @@ static void validate_timestamp(struct timespec *cur, int min_delay)
 	if (cur64 < start64 + min_delay || cur64 > start64 + max_delay) {
 		fprintf(stderr, "ERROR: %" PRId64 " us expected between %d and %d\n",
 				cur64 - start64, min_delay, max_delay);
-		test_failed = true;
+		if (!getenv("KSFT_MACHINE_SLOW"))
+			test_failed = true;
 	}
 }