Fix milestones-auth.js

[miketaylr]

Blocks #2106.

✓ firefox on darwin 17.4.0 - Milestones (auth) - Milestone editor opens then closes (clicks) (1.178s)
× firefox on darwin 17.4.0 - Milestones (auth) - Milestone editor opens then closes (key events) (26.022s)
    NoSuchElement: [POST http://localhost:4444/wd/hub/session/8b20d117-d125-9041-a854-ab680332b09c/element / {"using":"css selector","value":".wc-CategoryEditor-search"}] Unable to locate element: .wc-CategoryEditor-search
    For documentation on this error, please visit: http://seleniumhq.org/exceptions/no_such_element.html
    Build info: version: '3.5.2', revision: '10229a9020', time: '2017-08-21T17:54:21.164Z'
    System info: host: 'omg-5', ip: '192.168.2.83', os.name: 'Mac OS X', os.arch: 'x86_64', os.version: '10.13.3', java.version: '1.8.0_144'
    Driver info: driver.version: unknown
      at Test.Milestone editor opens then closes (key events) [as test]  <tests/functional/milestones-auth.js:55:10>
      at <src/lib/Test.ts:260:47>

✓ firefox on darwin 17.4.0 - Milestones (auth) - Clicking outside milestone editor closes it (7.104s)
✓ firefox on darwin 17.4.0 - Milestones (auth) - Clicking close button actually closes it? (5.951s)