Use admin/admin for testing, fixes T944

Summary: Keeps the same password "admin" after running tests. Reduces confusion.

Test Plan:
```
bin/behat --config application/tests/behat.yml -f progress
```

Reviewers: vladimir, aMoniker

Subscribers: rjmackay, srutto

Maniphest Tasks: T944

Differential Revision: https://phabricator.ushahidi.com/D463