Add tests for labelsetimage

Summary:
Fix wrong error output; reset labelsetimage

The labelsetimage needed to be reset in "TestRemoveLabels()" in order to
be correctly initialized. The test might have been correct but writing the image
to disk stored an incorrect labelsetimage (with the removed labels still present)

Add function to test label mask creation

In oder to test "LabelSetImage::CreateLabelMask",
a new test-function was added to the test suite.

Add function to test label erasure

In oder to test "LabelSetImage::EraseLabel" and
"LabelSetImage::EraseLabels" a new test-function was added to
the test suite.

Add and test function for single label removal

Modify function to test label merging

Modify existing function to test simultaneous merging of multiple labels.

Test Plan: start and perform the labelsetimage test

Reviewers: kislinsk, floca, O1 MITK Reviewer Group I

Reviewed By: floca, O1 MITK Reviewer Group I

Subscribers: kalali, floca, kislinsk

Maniphest Tasks: T28138

Differential Revision: https://phabricator.mitk.org/D639