The customizations for session tests to define custom workspaces and
configurations currently only cleanup the contents of the used test
directories if they are created by the customization implementation on
their own. When a path is passed from outside, e.g., injected via a
JUnit TempDir annotation, the folder is not cleaned up, but usually the
provider will not do that on its own. In particular, JUnit only removes
an injected temporary directory but not its contents.

With this change, the data created by session tests in custom workspace
or configuration folders is always cleaned up after the test, no matter
who created the temporary directory.

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