test: limit concurrency to 1

This was changed in Tap 13; we just upgraded 12 -> 14.

Parallelism causes issues with the way we use nock(),
which picks up requests from multiple tests running
concurrently, yielding confusing and wrong results.

The tests aren't super slow anyway, so just leave it off.

package.json

@@ -16,7 +16,7 @@
   "scripts": {
     "dump": "node test/dump.js",
     "start": "node demo/index.js",
-    "test": "npm run lint && tap ./test/*.test.js -R spec",
+    "test": "npm run lint && tap --jobs 1 ./test/*.test.js -R spec",
     "lint": "eslint -c .eslintrc lib"
   },
   "keywords": [