ui: Add dummpy / explanatory main function

This fixes regressions in the CI suite that fail otherwise unrelated
pull requests, probably due to nowadays stricter clippy errors (`cargo
+nightly clippy` previously failed with 'error[E0601]: `main` function
not found in crate `typed_html_tests`').

ui/main.rs

@@ -1,3 +1,7 @@
+fn main() {
+    println!("This crate is not meant to be run, it only serves as a compilation test.");
+}
+
 #[test]
 fn ui() {
     extern crate version_check;