Sonar lint fixes

[swalchemist]

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Mitigates a handful of SonarLint findings (codes for each are given in the commit messages). Also includes a new test to cover some of the modified code, and some related refactoring.

There are also a large number of changes to migrate some of the tests to JUnit 5 and improve how they assert on throwing an exception. Note that the @Test(expected = ... instances were migrated because this was required to use JUnit 5, but the JUnit 3 style exception assertions that use try/catch are not yet migrated, in order to reduce the volume of changes in this PR.

This was a draft PR until #679 was merged first.

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