Add new CI job for app tests on Focal

[kushaldas]

Status

Ready for review

Description of Changes

Fixes #5524

This adds related CI steps for app tests on Focal

Testing

• CI should have a new job for app tests on Focal.

Deployment

Any special considerations for deployment? Consider both:

1. Upgrading existing production instances.
2. New installs.

Checklist

If you made changes to the server application code:

• Linting (make lint) and tests (make test) pass in the development container

If you made changes to securedrop-admin:

• Linting and tests (make -C admin test) pass in the admin development container

If you made changes to the system configuration:

• Configuration tests pass

If you made non-trivial code changes:

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

Choose one of the following:

• I have opened a PR in the docs repo for these changes, or will do so later
• I would appreciate help with the documentation
• These changes do not require documentation

If you added or updated a code dependency:

Choose one of the following:

• I have performed a diff review and pasted the contents to the packaging wiki
• I would like someone else to do the diff review

[emkll]

Ci is passing and the docker caching is working in CI. LGTM

[emkll]

we can probably re-use the same cache dir as for Xenial just specifying different layer files