Update test file (#302)

# Summary
Updates the pretest script to copy `airtable.browser.js` into `test/test_files`.

# Testing
- `npm run test`
- From `test/test_files` run `python2 -m SimpleHTTPServer` and make sure it still works as before.

package.json

@@ -6,7 +6,7 @@
   "repository": "git://github.com/airtable/airtable.js.git",
   "private": false,
   "scripts": {
-    "pretest": "npm run lint && npm run prepare; true",
+    "pretest": "npm run lint && npm run prepare && cp build/airtable.browser.js test/test_files; true",
     "lint": "eslint '*/**/*.{js,ts,tsx}'",
     "format": "prettier --write '**/*.js'",
     "test": "jest --env node --coverage --no-cache",
@@ -72,4 +72,4 @@
   "engines": {
     "node": ">=8.0.0"
   }
-}
+}