Refactor JUnit tests to use JUnit 5

[DPUkyle]

... and remove JUnit 4 dependency

[DPUkyle]

I briefly experimented with this, even removing the JUnit 4 jar entirely. The loss of JUnit 4's @Rule combined with the TemporaryFolder type is a pretty major setback.

Some alternative ideas:

• Use the documented migration libraries, though this is just a temporary fix
• Use Spock's @TempDir annotation on a File or Path
• @TempDir, but applied to a FileSystemFixture. This allows a cool DSL to define folder structure and files, but the file contents must be set in-line. The changes would end up being pretty extensive.

Regardless, I'd want to spend some more time thinking about how to best refactor this code - and if the effort is worth the return. More research is needed.