test: spawn_wait() starts a non-RPC Nvim process

Problem:
Can't use `n.clear()` to test non-RPC `nvim` invocations. So tests end
up creating ad-hoc wrappers around `system()` or `jobstart()`.

Solution:
- Introduce `n.spawn_wait()`
- TODO (followup PR): Rename `n.spawn()` and `n.spawn_wait()`.
  It's misleading that `n.spawn()` returns a RPC session...

src/nvim/main.c

@@ -634,6 +634,7 @@ int main(int argc, char **argv)
   if (params.luaf != NULL) {
     // Like "--cmd", "+", "-c" and "-S", don't truncate messages.
     msg_scroll = true;
+    DLOG("executing Lua -l script");
     bool lua_ok = nlua_exec_file(params.luaf);
     TIME_MSG("executing Lua -l script");
     if (msg_didout) {