This migrates the SaveManagerTest to JUnit 5, using the
SessionTestExtension with the CustomSessionWorkspace. It consists of the
following changes:
- the individual test suites in the subclasses of SaveManagerTest were
unused and are thus removed
- further unused functionality in those subclasses is removed
- JUnit 4 dependencies are replaced with JUnit 5 dependencies in the
plug-in org.eclipse.core.tests.resources.saveparticipant
- the plug-in org.eclipse.core.tests.resources.saveparticipant is
declared as a test plug-in and tests are enabled for the Maven build
- the SaveParticipantTest, which executed the SaveManagerTest from the
central org.eclipse.core.tests.resources plug-in, is removed as it was
not part of any test suite (and thus not executed) anyway

Contributes to
https://github.com/eclipse-platform/eclipse.platform/issues/903