Migrate fully to JUnit 5

[dlurton]

• There are many tests still using JUnit 4.
• There are now many test classes which mix JUnit 5's annotations and JUnit 4's assertions, which is messy.
• JUnit 4 tests don't seem to be running parallel under JUnit 5's backward compatibility runner.

One challenge: we make somewhat heavy use of junitparams, which is tied to JUnit 4 and isn't needed any longer (since JUnit5 has such parameterization built in), so some non-trivial amount of mechanical refactoring will be required.

This task will be completed when all references to JUnit 4 (and junitparams) are removed from the code in this repository.

Also, consider if #576 is required prior to starting this issue. Should all tests should import their assert* functions instead of inheriting them from TestBase.