Migrates the tests in org.eclipse.core.tests.resources.refresh to JUnit
5. Replaces test annotations and migrates assertions.

The change also improves test isolation for the RefreshProviderTest. The
tests could not be executed individually, because modified refresh
settings are not followed by a wait for a potentially pending refresh
request and resetting the TestRefreshProvider is performed at before
instead of after changing the refresh settings.

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