[TASK] Add unittests for t3lib_div::formatSize

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

Change-Id: If9466ebdf9d7177de298db14c5460cddfcc5cbe1
Resolves: #37297
Releases: 6.0
Reviewed-on: http://review.typo3.org/11296
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog