Upgrade from JUnit 4 to JUnit 5

We are still using JUnit 4, while JUnit 5 has been available for a long time, and has many nice features that we could benefit from, such as:

• assertThrows in the test method, rather than as an annotation, allowing to test multiple throws in a single test.
• Nice functionality for parameterized tests.

This will require:

• Project metadata changes (manifests, .classpath, etc).
• Imports to org.junit.jupiter.* instead of org.unit.*.
• Some changes to our Maven build configuration (surefire).
• Carefully inspecting all assertion statements, due to backward incompatible API changes, such as:
  • org.junit.Assert: public static void assertEquals(String message, Object expected, Object actual)
  • org.junit.jupiter.api.Assertions: public static void assertEquals(Object expected, Object actual, String message)
  • If you do use assertEquals on 3 String-typed arguments, that will still compile with the new import, but will actually mix the arguments.
• ...

We do the upgrade in two steps:

• Upgrade JUnit tests (See !601 (merged))
• Upgrade JUnit Plug-in tests and setext.texteditor test, we might need Tycho 4.0 for this (see !601 (comment 1147222)).