Relax timing test in the other direction.

On my test system under MSYS (Cygwin64), a period of 991ms was
measured after a sleep of 1s.

test_timing.c

@@ -29,7 +29,7 @@ int main(int argc, char *argv[])
 	time_get(&b);
 	time_sub(&b, &a, &c);
 	printf("Measured: %ums\n", time_as_ms(&c));
-	assert(time_as_ms(&c) >= 999);
+	assert(time_as_ms(&c) >= 950);
 	assert(time_as_ms(&c) <= 1050);
 	printf("Starting 3s timeout\n");
 	timeout_start(&to, 3000);