Add failing test using internal ember testing tools

packages/@ember/-internals/glimmer/tests/integration/custom-helper-test.js

@@ -0,0 +1,78 @@
+import { RenderingTestCase, moduleFor, strip } from 'internal-test-helpers';
+
+import { precompileJSON } from '@glimmer/compiler';
+import { getTemplateLocals } from '@glimmer/syntax';
+import { createTemplateFactory } from '@ember/template-factory';
+
+import { Helper } from '../../index';
+import { setComponentTemplate } from '@glimmer/manager';
+import { templateOnlyComponent } from '@glimmer/runtime';
+
+/**
+ * The template-compiler does not support strict mode at this time.
+ * precompile from ember-template-compiler returns a string and
+ * not a template-factory, so it doesn't help with strict-mode testing.
+ *
+ * We also can't import from `@ember/template-compiler` because it
+ * doesn't exist to this kind of test, otherwise we'd be able to use
+ * precompileTemplate, which would be perfect :D
+ *
+ * Copied(ish) from https://github.com/NullVoxPopuli/ember-repl/blob/main/addon/hbs.ts#L51
+ */
+function precompileTemplate(source, { moduleName, scope = {} }) {
+  let locals = getTemplateLocals(source);
+
+  let options = {
+    strictMode: true,
+    moduleName,
+    locals,
+    isProduction: false,
+    meta: { moduleName },
+  };
+
+  // Copied from @glimmer/compiler/lib/compiler#precompile
+  let [block] = precompileJSON(source, options);
+
+  let blockJSON = JSON.stringify(block);
+  let templateJSONObject = {
+    id: moduleName,
+    block: blockJSON,
+    moduleName: moduleName ?? '(unknown template module)',
+    scope,
+    isStrictMode: true,
+  };
+
+  let factory = createTemplateFactory(templateJSONObject);
+
+  return factory;
+}
+
+moduleFor(
+  'Custom Helper test',
+  class extends RenderingTestCase {
+    ['@test works with strict-mode']() {
+      class Custom extends Helper {
+        compute([value]) {
+          return `${value}-custom`;
+        }
+      }
+
+      let template = strip`
+        {{ (Custom 'my-test') }}
+      `;
+
+      let precompiled = precompileTemplate(template, {
+        moduleName: 'strict-mode',
+        scope: () => ({ Custom }),
+      });
+
+      let strictMode = setComponentTemplate(precompiled, templateOnlyComponent());
+
+      this.render(`<this.strictMode />`, {
+        strictMode,
+      });
+      this.assertText('my-test-custom');
+      this.assertStableRerender();
+    }
+  }
+);