GitHub Actions for testing PRs fail to run

[JostMigenda]

Tests currently fail to run; apparently because they request Ubuntu 18.04, which is no longer supported. See screenshot for an error message of the most recent test run on #75.

@LouiseABowler, you offered to look into this at Wednesday’s meeting, but let me know if I can help!

[JostMigenda]

After the discussion at the meeting this morning, I started addressing this in 9471bd8 (part of #75) and then noticed a few other low-hanging fruit, which I’ve fixed in #78. See discussion at the latter PR for the current status.