add an entry point for theme shared testing

npm/ng-packs/packages/theme-shared/testing/ng-package.json

@@ -0,0 +1,7 @@
+{
+  "$schema": "../../../node_modules/ng-packagr/ng-package.schema.json",
+  "dest": "../../dist/theme-shared/testing",
+  "lib": {
+    "entryFile": "src/public-api.ts"
+  }
+}