Feature: #60123 - Unit base test case removes test files

See Issue #60123

Description

Some unit tests need to create test files or directories to check the system under test. Those files should be removed again. A test can now register absolute file paths in $this->testFilesToDelete, and the generic tearDown() method will then remove them. Only files, links and directories within typo3temp/ are allowed.

Impact

This allows tests to clean up the environment without leaving obsolete test files behind.