[dogfooding][cleanup] Use static inner class clean up for
test code

Using this cleanup brings:
- It reviews the feature
 - For optimization: small memory usage reduction as the outer pointer
is removed


Change-Id: Id3b4fcee22e52183fd8f00740e0ba4d152954e1c
Signed-off-by: Lars Vogel <Lars.Vogel@vogella.com>
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.text/+/179071
Tested-by: Platform Bot <platform-bot@eclipse.org>