Currently, the SessionTestExtension with all its customizations is not
only instantiated in the test-executing host application but also in
every remote Eclipse application that is started to run a test case in
an isolated session. Even though the current implementation already
considers this during execution of the extensions and accesses to the
customizations, it would be preferable to even only instantiate them if
required.

This change introduces dummy implementations for the session test
customizations that are used during remote execution to avoid
unnecessary overhead.

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