add production mode tests

Due to the inability to detect warnings in REPL tests, I implemented running something very similar, but with the -prod flag enabled.
(See https://github.com/vlang/v/pull/2536)

There is also a minor change in os.walk_ext to not add duplicated path separators:
/path//file.ext -> /path/file.ext

vlib/os/os.v

@@ -856,11 +856,12 @@ pub fn walk_ext(path, ext string) []string {
 	}
 	mut files := os.ls(path) or { panic(err) }
 	mut res := []string
+	separator := if path.ends_with(path_separator) { '' } else { path_separator}
 	for i, file in files {
 		if file.starts_with('.') {
 			continue
 		}
-		p := path + path_separator + file
+		p := path + separator + file
 		if os.is_dir(p) {
 			res << walk_ext(p, ext)
 		}