chore(deps): update dependency nightwatch to v2.6.15

Author: renovate[bot]

package.json

@@ -100,7 +100,7 @@
     "jest-mock-warn": "^1.1.0",
     "lint-staged": "13.1.2",
     "mockdate": "^3.0.5",
-    "nightwatch": "2.6.13",
+    "nightwatch": "2.6.15",
     "nightwatch-helpers": "^1.2.0",
     "prettier": "2.8.3",
     "rollup": "2.79.1",