Fix test_rm failure on Focal

[kushaldas]

Status

Ready for review

Description of Changes

Fixes #5601

On Focal /bin/shred exists, that was reason for the failure. /sbin
does not contain the shred executable in both Xenial and Focal.

Testing

• CI should be green
• To test on focal, first create a local dummy branch from dev_focal and rebase on top of Updates pytest pluggy testinfra molecule and the universe #5585
• git rebase -i updates_pytest_pluggy_testinfra_molecule_and_the_universe
• BASE_OS=focal securedrop/bin/dev-shell bin/run-test -s -v tests/test_rm.py

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

[rmol]

This does fix the error on Focal. You've made me realize that PATH could just contain config.TEMP_DIR, which would insulate against any future changes, but there's also a typo in the test script a little later which only accidentally produces the correct result, so I'm going to make both changes in a separate PR.