test: install plugin in fixture directory

Author: mrstork

tests/fixtures/angular-21/package-lock.json

@@ -32,6 +32,7 @@
     "@angular/platform-server": "^21.0.0-next.0",
     "@angular/router": "^21.0.0-next.0",
     "@angular/ssr": "^21.0.0-rc.1",
+    "@netlify/angular-runtime": "file:../../..",
     "express": "^5.1.0",
     "rxjs": "~7.8.0",
     "tslib": "^2.3.0"
@@ -46,4 +47,4 @@
     "typescript": "~5.9.2",
     "vitest": "^4.0.0"
   }
-}
+}