Upgrade to JUnit 5

This MR:

• removes the JUnit 4 dependencies from spring itself
• adds JUnit 5 dependencies
• changes all tests from JUnit 4 tests to JUnit 5

Closes #331 (closed)

approved this merge request

started a merge train

mentioned in merge request !333 (merged)

mentioned in commit 3adba076

merged

unassigned @tnulle