[BUGFIX] Fix unit tests for TypeConverter classes

The patch Ia4ec3a6c6f5bc6c07717a4956ab12e3e779c3c5a didn't adjust the
unit tests. This fix adds the missing mocks.

Resolves: #49803
Releases: master, 6.2
Change-Id: I829e62eaf3b172edeb295da57093b424edb13d40
Reviewed-on: http://review.typo3.org/33330
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>