[TASK] Remove AbstractPluginTest

AbstractPlugin ("Pi1" FE plugins) has been
marked as "not maintained anymore" a while
ago.

The unit tests are a heavily mocking aweful
mess, most of them look at details that are
obvious reading the implementation. The
central topics like the ContentObjectRenderer
have unit tests on their own to sort out details.

AbstractPlugin may see a final deprecation
and there are little code changes in general.

The patch removes the "unit" tests to reduce
maintenance nightmares in this area, the tests
don't add much value.

Resolves: #100408
Related: #100249
Releases: main
Change-Id: If906aae54f1fdcf55b7aa88486e28f276322df6d
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78378


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: core-ci <typo3@b13.com>