[CLEANUP] Replace "fixture" with "subject" in the unit tests

"fixture" for the instance of the class to test has been a misnomer.
("Fixture" usually refers to the code used to set up the testing
environment.)

Resolves: #53518

Change-Id: I713f1561ebfd43c1edb3c1fe622e5103815935c3
Reviewed-on: https://review.typo3.org/25259
Reviewed-by: Oliver Klee
Tested-by: Oliver Klee