explicitly add @ember/test-helpers to devDependencies in test-packages

Author: Gaurav0

test-packages/basic-app/package.json

@@ -26,6 +26,7 @@
   },
   "devDependencies": {
     "@ember/optional-features": "^1.3.0",
+    "@ember/test-helpers": "^2.6.0",
     "@glimmer/component": "^1.1.2",
     "@glimmer/tracking": "^1.0.0",
     "babel-eslint": "^10.1.0",

test-packages/custom-fastboot-app/package.json

@@ -23,6 +23,7 @@
   },
   "devDependencies": {
     "@ember/optional-features": "^1.3.0",
+    "@ember/test-helpers": "^2.6.0",
     "@glimmer/component": "^1.1.2",
     "@glimmer/tracking": "^1.0.0",
     "babel-eslint": "^10.1.0",

test-packages/custom-sandbox-app/package.json

@@ -21,6 +21,7 @@
   },
   "devDependencies": {
     "@ember/optional-features": "^1.3.0",
+    "@ember/test-helpers": "^2.6.0",
     "@glimmer/component": "^1.1.2",
     "@glimmer/tracking": "^1.0.0",
     "babel-eslint": "^10.1.0",

test-packages/ember-cli-fastboot-testing-app/package.json

@@ -22,6 +22,7 @@
   },
   "devDependencies": {
     "@ember/optional-features": "^1.3.0",
+    "@ember/test-helpers": "^2.6.0",
     "babel-eslint": "^10.1.0",
     "broccoli-asset-rev": "^3.0.0",
     "ember-auto-import": "^1.5.3",