Mega: End-to-end test the TinyPilot Debian package install

[mtlynch]

One of the things we lost when we vanquished Ansible from our install process (#1596) was that Ansible tested the idempotence of a TinyPilot install.

Currently, changes to our installer are error-prone and time-consuming to test (see regression of #1615). It's worth investing in some e2e testing scripts to make sure the install works as we expect.

Notes

The rough idea I have in mind is:

1. Create a Dockerfile that inherits from debian:bullseye and makes whatever changes to make our install work (e.g., we'll probably have to populate a /boot/config.txt file with dummy data)
2. Build that Dockerfile in CI and run it on an ARM instance
3. Run a script that runs through different install scenarios to verify that our install does what we expect

Things to test

• Verify that changes to config files have been made (and verify that expected lines appear exactly once)
  • e.g. to /boot/config.txt and /etc/modules
• Verify that tinypilot/ustreamer user accounts are created
• Verify that TC358743-based installs work as expected
• Verify that MS2109-based installs work as expected
• Verify that install, uninstall, reinstall has the same effect as single install (prevent regression of Prevent tinypilot.postinst from appending to Janus config #1615)

[mtlynch]

Forgot we already had this in #1659. Closing as dupe.