test: add a bit more testing for vim.on_key() (#28095)

Also:
- Don't use NUMBUFLEN as buffer length as its unrelated.
- Restore accidentally removed comment from last commit.

src/nvim/lua/executor.c

@@ -2066,7 +2066,7 @@ char *nlua_register_table_as_callable(const typval_T *const arg)
 
 void nlua_execute_on_key(int c)
 {
-  char buf[NUMBUFLEN];
+  char buf[MB_MAXBYTES * 3 + 4];
   size_t buf_len = special_to_buf(c, mod_mask, false, buf);
 
   lua_State *const lstate = global_lstate;