Migrate to JUnit 5

JUnit 5 has a ton of neat features that make writing tests easier and more expressive. It provides a compatibility layer for JUnit 4 and 3, so we should be able to migrate pretty easily there.

In order to migrate v4 tests to v5, we'll need to support the following:

• LoggerContextRule should have an equivalent ParameterResolver extension which injects a LoggerContext parameter among other types. (This class should remain for JUnit 4 users, and an abstraction might help in making something equivalent available for JMH tests as well).
  • This is currently implemented via the LoggerContextSource annotation.
• Test categories should be converted to tags.
• @Ignore should be replaced with @Disabled or appropriate conditional disabled annotations (also useful for replacing assumeThat() tests).
  • IntelliJ's convert to JUnit 5 refactoring action does this automatically.
• Hamcrest must be added as an explicit dependency and/or replaced by something better like AssertJ.
  • IntelliJ's convert to JUnit 5 refactoring action does this automatically.
• The other test annotations need to be updated to the new ones.
  • IntelliJ's convert to JUnit 5 refactoring action does this automatically.
• Parameterized tests can use the new parameters annotations to simplify things a lot.
• TemporaryFolder rule can be replaced with @TempDir parameter injection (experimental feature).
• Other rule usage needs to be converted to use appropriate v5 features or turned into extensions.

Note that this upgrade should be backported to the release-2.x branch after Java 8 is made the baseline there.