[TASK] WincacheBackend: Add a "Testing"-context to the tests

Resolves: #46817
Related: #46768
Releases: 6.0
Change-Id: I5ebc5f026d92ae8070c2dbfd4679e2889bad6c07
Reviewed-on: https://review.typo3.org/19459
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn