Remove aad from tests

[pdehaan]

We removed the file sha256 hash in #470, but looks like we may have a few lingering references in the test/** directory: https://github.com/mozilla/send/search?utf8=%E2%9C%93&q=aad&type=

1. /test/frontend/frontend.test.js:182 — aad: arrayToHex(new Uint8Array(fileHash)),
2. /test/frontend/frontend.test.js:364 — aad: 'some_bad_hashz',
3. /test/server/server.test.js:49 — aad: '11111',