[Tests] add posttest using npx aud to run npm audit without a lockfile

Author: ljharb

.travis.yml

@@ -36,6 +36,8 @@ matrix:
   include:
     - node_js: "lts/*"
       env: PRETEST=true
+    - node_js: "lts/*"
+      env: POSTTEST=true
     - node_js: "4"
       env: COVERAGE=true
     - node_js: "12.9"

package.json

@@ -49,6 +49,7 @@
         "pretest": "npm run --silent readme && npm run --silent lint",
         "test": "npm run --silent coverage",
         "tests-only": "node test",
+        "posttest": "npx aud",
         "readme": "evalmd README.md",
         "postlint": "eclint check * lib/* test/*",
         "lint": "eslint lib/*.js test/*.js",