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/hello_main.c

@@ -0,0 +1,3 @@
+int main() {
+    return 1;
+}