testVerticallyMergedCellBorder failing on some systems

[Ilmari Lauhakangas]

Someone I am mentoring got a test failure on Windows for this test:
CPPUNIT_TEST_NAME="testVerticallyMergedCellBorder" make 
CppunitTest_sw_core_layout

I tried it myself on Linux and it is failing with

[CUT] sw_core_layout
[_RUN_____] testVerticallyMergedCellBorder::TestBody
E: lt_string_value: assertion `string != ((void *)0)' failed
E: lt_string_value: assertion `string != ((void *)0)' failed
E: lt_string_value: assertion `string != ((void *)0)' failed
E: lt_string_value: assertion `string != ((void *)0)' failed
vmerge-cell-border.docx:
/home/ilmari/libreoffice/sw/qa/core/layout/layout.cxx:499:testVerticallyMergedCellBorder::TestBody
assertion failed
- Expression: bFound
- Bad vertical position for border point: 3659 Expected positions: 3094, 
3643, 4193, 4743

It was added in 
https://git.libreoffice.org/core/commit/66ac8e60896f6306bed8fbb34606fd14474f19ce

On Windows the failure was

C:/cygwin/home/user/lode/dev/core/sw/qa/core/layout/layout.cxx:493:testVerticallyMergedCellBorder::TestBody
assertion failed
- Expression: bFound
- Bad vertical position for border point: 4594 Expected positions: 3022, 
3540, 4059, 4578

Ilmari