Make ember-lts-4.12 compat scenario runnable

Three stacked runtime issues were blocking ember-lts-4.12 once the
`require` import issue from ember-cli#321 cleared. All three affect the compat
path (ENABLE_COMPAT_BUILD=true, ember4 deps overrides) and are hidden
for ember-source >= 6 because the upstream code was rewritten.

1. "Cannot read properties of undefined (reading 'has')" at
   `makeComputedDecorator` → `COMPUTED_GETTERS.has(...)`.

   ember-source <= 5.x's `@ember/-internals/metal` has
   `let COMPUTED_GETTERS; if (DEBUG) { COMPUTED_GETTERS = new WeakSet(); }`
   paired with `assert(..., !COMPUTED_GETTERS.has(...))`. `DEBUG` comes from
   `@glimmer/env`, whose published default is `DEBUG = false`. Without
   babel-plugin-debug-macros processing ember-source, the init is
   tree-shaken while the `assert` predicate still evaluates eagerly, so
   `COMPUTED_GETTERS` is undefined at runtime.

   Alias `@glimmer/env` to a local shim that exports `DEBUG = true` so the
   init and the use stay consistent (ember-source >= 6 switched to
   `isDevelopingApp()` from `@embroider/macros`, which `buildMacros()`
   already handles — hence the problem is specific to the older Ember
   versions exercised by the compat scenarios).

2. "ReferenceError: requirejs is not defined" from ember-qunit's
   auto-loadTests.

   ember-qunit@8's `start()` calls `loadTests()` which uses
   `requirejs.entries` — there's no loader.js in a vite build.
   ember-qunit@9 dropped that call entirely. Pass `{ loadTests: false }`
   so `start()` skips the requirejs-based loader; tests/index.html already
   uses `import.meta.glob` to discover them. The option is safely ignored
   by ember-qunit 9.

3. "Cannot read properties of undefined (reading
   '_APPLICATION_TEMPLATE_WRAPPER')" from `@ember/test-helpers`'s `visit`
   and `setup-rendering-context`.

   test-helpers 4.x reads `global.EmberENV._APPLICATION_TEMPLATE_WRAPPER`
   to decide whether to look inside the `.ember-view` wrapper. In classic
   ember-cli apps the global is populated from index.html; here we need
   to set it ourselves from config. Assign `globalThis.EmberENV` in
   `test-helper.js`. test-helpers 5.x no longer reads this at all, so the
   assignment is harmless for non-compat scenarios.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

Author: NullVoxPopuli

test-app/glimmer-env-shim.mjs

@@ -0,0 +1,10 @@
+// Force DEBUG=true for test builds. ember-source <= 5.x uses
+// `import { DEBUG } from '@glimmer/env'` paired with `if (DEBUG)`-guarded
+// init (e.g. COMPUTED_GETTERS = new WeakSet()) alongside `assert()` calls
+// whose predicate reads those same variables. Without
+// babel-plugin-debug-macros processing these, the default export
+// `DEBUG = false` from @glimmer/env tree-shakes the init away while the
+// assert still runs, producing "Cannot read properties of undefined
+// (reading 'has')" at runtime.
+export const DEBUG = true;
+export const CI = false;

test-app/tests/test-helper.js

@@ -3,11 +3,13 @@ import config, { enterTestMode } from 'test-app/config/environment';
 import { setApplication } from '@ember/test-helpers';
 import { start as qunitStart, setupEmberOnerrorValidation } from 'ember-qunit';
 
+globalThis.EmberENV = config.EmberENV;
+
 export function start() {
   enterTestMode();
   setApplication(Application.create(config.APP));
 
   setupEmberOnerrorValidation();
 
-  qunitStart();
+  qunitStart({ loadTests: false });
 }

test-app/vite.config.mjs

@@ -1,6 +1,7 @@
 import { defineConfig } from 'vite';
 import { extensions, ember, hbs, classicEmberSupport } from '@embroider/vite';
 import { babel } from '@rollup/plugin-babel';
+import { fileURLToPath } from 'node:url';
 
 export default defineConfig({
   plugins: [
@@ -11,4 +12,11 @@ export default defineConfig({
       extensions,
     }),
   ],
+  resolve: {
+    alias: {
+      '@glimmer/env': fileURLToPath(
+        new URL('./glimmer-env-shim.mjs', import.meta.url),
+      ),
+    },
+  },
 });