[441172] Extend findMasterEquivalence to not only check eOpposites

Currently the findMasterEquivalence method only considers equivalences
whose reference is the eOpposite of the given diff. But sometimes the
equivalence's reference is the same as the one given by the diff. The
findMasterEquivalence method is extended to handle these cases too.

Includes testcases for this issue.

Bug: 441172
Signed-off-by: Stefan Dirix <sdirix@eclipsesource.com>
Also-by: Laurent Goubet <laurent.goubet@obeo.fr>
Change-Id: Ic3678939c0798c9993c41d329f6c0d4e4d098c27