Route tests through a #testing alias, ban ../ imports, and document the alias ownership rule

src/runtime.test.ts

@@ -7,7 +7,7 @@ import { assert, expect, test } from "vite-plus/test";
 import { output } from "./command.ts";
 import { prepareRuntime } from "./runtime.ts";
 import type { RuntimeProfile } from "./runtime.ts";
-import { IS_ROOT, makeTempDir } from "./testing.ts";
+import { IS_ROOT, makeTempDir } from "#testing";
 
 const MASTER_KEY = "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8";
 const ENCRYPTED = "v1:ICEiIyQlJicoKSor:oV_FAgnsN3p1FyegJxwGVdyjfyPio6qDeSUwbA";