[TASK] Add unittests for array related functions in t3lib_div

Some of the most basic functions should be covered with unit tests.

The change creates tests for inArray, array_merge, remapArrayKeys
and keepItemsInArray.

Change-Id: Id57148bc0d0418ed7308b9647c63af2d82ec94f2
Resolves: #37299
Releases: 6.0
Reviewed-on: http://review.typo3.org/11298
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog