Failure in VclTextTest::testSimpleText due to antialiasing (Linux)

[Ilmari Lauhakangas]

A mentee is running into a test failure on Ubuntu 21.10 (output below). 
The image produced by the test has a coloured fringe. Luboš commented on it:

"Colors in AA mean that's AA for LCD.
There's special code to handle that, I suppose the test should 
temporarily disable that for the text rendering."

How can we achieve this?

Ilmari

[CUT] vcl_svm_test
[_RUN_____] (anonymous namespace)::CanvasBitmapTest::runTest
(anonymous namespace)::CanvasBitmapTest::runTest finished in: 144ms
[_RUN_____] VclComplexTextTest::testArabic
VclComplexTextTest::testArabic finished in: 107ms
[_RUN_____] VclComplexTextTest::testKashida
VclComplexTextTest::testKashida finished in: 1ms
[_RUN_____] VclComplexTextTest::testTdf95650
VclComplexTextTest::testTdf95650 finished in: 5ms
[_RUN_____] VclComplexTextTest::testCaching
VclComplexTextTest::testCaching finished in: 8ms
[_RUN_____] VclTextTest::testSimpleText
/home/akshit16/libreoffice/vcl/qa/cppunit/text.cxx:251:VclTextTest::testSimpleText
double equality assertion failed
- Expected: 28
- Actual  : -1
- Delta   : 2

VclTextTest::testSimpleText finished in: 27ms
[_RUN_____] VclTextTest::testVerticalText
VclTextTest::testVerticalText finished in: 0ms
[_RUN_____] VclTextTest::testTextLayoutCache
VclTextTest::testTextLayoutCache finished in: 0ms
[_RUN_____] VclTextTest::testImplLayoutRuns_AddPos
VclTextTest::testImplLayoutRuns_AddPos finished in: 0ms
[_RUN_____] VclTextTest::testImplLayoutRuns_AddRuns
VclTextTest::testImplLayoutRuns_AddRuns finished in: 0ms
[_RUN_____] VclTextTest::testImplLayoutRuns_PosIsInRun
VclTextTest::testImplLayoutRuns_PosIsInRun finished in: 0ms
[_RUN_____] VclTextTest::testImplLayoutRuns_PosIsInAnyRun
VclTextTest::testImplLayoutRuns_PosIsInAnyRun finished in: 0ms
[_RUN_____] VclTextTest::testImplLayoutArgsBiDiStrong
VclTextTest::testImplLayoutArgsBiDiStrong finished in: 0ms
[_RUN_____] VclTextTest::testImplLayoutArgsBiDiRtl
VclTextTest::testImplLayoutArgsBiDiRtl finished in: 0ms
[_RUN_____] VclTextTest::testImplLayoutArgsRightAlign
VclTextTest::testImplLayoutArgsRightAlign finished in: 0ms
[_RUN_____] 
VclTextTest::testImplLayoutArgs_PrepareFallback_precalculatedglyphs
VclTextTest::testImplLayoutArgs_PrepareFallback_precalculatedglyphs 
finished in: 10ms
text.cxx:251:Assertion
Test name: VclTextTest::testSimpleText
double equality assertion failed
- Expected: 28
- Actual  : -1
- Delta   : 2