[TASK] Add unit tests to new autoloader XCLASS handling

Add missing tests to the autoloader test case to test the new XCLASS
handling of issue #31893. The patch also fixes a minor error when writing
registered XCLASS files to the autoloader cache file. This error is
covered by the third added test.

Change-Id: I394c05a48ee2a2796299e72b17840689432212a2
Resolves: #36690
Related: #31893
Releases: 6.0
Reviewed-on: http://review.typo3.org/10843
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn