fix(test_downstream_projects): don't double build core while testing downstreams

Author: christopherthielen

package.json

@@ -12,7 +12,7 @@
     "fixmaps:bundle": "tweak_sourcemap_paths -a --include '_bundles/**/*.js.map'",
     "prepublishOnly": "npm run build",
     "test": "karma start",
-    "test:downstream": "npm run build && test_downstream_projects",
+    "test:downstream": "test_downstream_projects",
     "docs": "generate_docs",
     "docs:publish": "generate_docs && publish_docs",
     "watch": "run-p watch:*",