gbasm: return the position of the first differing byte

This aids debugging failing tests by showing exactly where things
failed in assertions that follow failed comparisons.

src/gbasm/tests/test.c

@@ -21,7 +21,7 @@ static int cmp_mem(const uint8_t *test_data, const uint8_t *expected_data, size_
 		if (test_data[i] != expected_data[i]) {
 			g_test_message("memory contents differed at index %zu, data=%x, expected=%x",
 			               i, test_data[i], expected_data[i]);
-			return -1;
+			return i;
 		}
 	}