workaround for rounding errors when handling merged cells (fdo#38414)

Change-Id: I4d36e4b86c77a7356a8c221cbfc5735e925392ba
add messages to utility test functions

So that e.g. parseDump() in case of an incorrect xpath does not just
plain give an equality assertion failure, which on its own is confusing.

Change-Id: Ib9fc2b4f6720b5c472773afbe8dc18cd8961465c
testcase for fdo#38414

Change-Id: I9467ae2ba5155175d5be8218248edada136a5ca1