(bug 51363) Add more tests to for bad diffs.

This adds high level tests for the fixes for bad diffs in change objects.

REQUIRES: I8ddc15d9 in Diff.
REQUIRES: I3835156f in WikibaseDataModel

Change-Id: I415a8b5c6aa3ff8894647d9ecf126181a3515ea7
Merge "(bug 51363) Add more tests to for bad diffs."