[xenial] fix 2FA unit tests

[redshiftzero]

Status

Ready for review

Description of Changes

Fixes #4037

Changes proposed in this pull request:

• Runs teardown if there's an issue with the driver setup (a common issue during functional test setup)
• Also re-uses the hardening fixture in the 2FA app tests, separated into another commit for ease of review
• See my comment here for more details on the underlying issue.

Testing

• CI trusty test job should pass
• 2FA tests should pass in CI xenial test job

Deployment

This touches test only code.

Checklist

If you made changes to the server application code:

• Linting (make ci-lint) and tests (make -C securedrop test) pass in the development container

If you made non-trivial code changes:

• I have written a test plan and validated it for this PR