[455255] Fix determination of TargetFeature in DiffUtil

The method getTargetFeature in DiffUtil is fixed to return the correct
feature when the DifferenceSource is RIGHT. This avoids ordering issues
as well as NullPointer Exceptions.

Includes a testcase.

Bug: 455255
Signed-off-by: Stefan Dirix <sdirix@eclipsesource.com>
Change-Id: Ie2b4e744cec5d297461f6a9c9127b9400a24d10a