add tests.sh, shellcheck and fix compare_versions

[osodracnai]

This commits adds the tests.sh file with unit tests for helper functions in upgrade.sh, fix some shellcheck warnings

[brandond]

Couple nits

Long term I would like to make this script consistent about using ${FOO} instead of $FOO for everything but positional arguments ($1 and so on) - right now we alternate between the two for no particular reason. But we don't need to fix that now.

[dereknola]

Busybox likely lacks the necessary libraries to support the checkout action, so you may need to manually craft a git clone command for it. Or, checkout the repo in a previous step on the ubuntu host, then store them as artifacts and have busybox pull those down in the next stage. See https://docs.github.com/en/actions/using-workflows/storing-workflow-data-as-artifacts

[dereknola]

See actions/checkout#334 (comment), where it was discussed. its possible they fixed this in v3, but maybe not.