Various quality-of-life improvements (ideas from @33577 )

* Very basic `hello_main.c`
* Big starting hint to `write_c`
* Better error messages on failed test
  * TODO: also improve it for asmgen, but not sure how to do it in a good way

examples/c/comma.c

@@ -1,5 +1,4 @@
-int main()
-{
+int main() {
     int y = 2;
     int x = (y += 2, 2, y + 3);
     return x == 7;