Migrate tests to JUnit5

[strangelookingnerd]

Migrates tests to JUnit5. Changes include

• Updated JUnit dependency
• Replaced usage of junit.framework.* for the JUnit5 equivalents
• Flipped order of parameters for some assertions comparing the expected vs. actual values
• Simplified some assertions, e.g. assertEquals(null, object) becomes assertNull(object)

There would be plenty of other possible improvements and best-practises that were not implemented in order to keep the changeset minimal. If desired I could add these as well in a separate PR.

Verified with mvn clean verify that build passes before and after my changes.
Overall number of tests and coverage did not change. Note that org.apache.commons.jxpath.ri.model.jdom.JDOMModelTest#testID did not do anything so I chose to use @Disabled with a meaningful message.

[garydgregory]

@strangelookingnerd
Merged! TY. One task I'd like to see done is using assertThrows instead of custom try-catch blocks.