Help background fill autest to run on ubuntu

[duke8253]

this resolves #10862

[moonchen]

I'm getting some failures in on Ubuntu 23.04 running in UTM.

results2.txt

ATS is built as follows:

CC=clang-17 CXX=clang++-17 cmake -GNinja -DCMAKE_INSTALL_PREFIX:PATH=/opt/ats2 \
    -DCMAKE_BUILD_TYPE=Debug \
    -DENABLE_CCACHE=ON \
    -DENABLE_ASAN=OFF \
    -DCMAKE_EXPORT_COMPILE_COMMANDS=ON \
    -DENABLE_AUTEST=ON \
    ..

cmake --build .

cmake --install .

Calling autest:

./autest.sh --filter=background_fill --ats-bin=/opt/ats2/bin | sed -r "s/\x1B\[([0-9]{1,3}(;[0-9]{1,2};?)?)?[mGK]//g" > /tmp/results2 2>&1

[shinrich]

Applying this patch make background_fill test pass in my ubuntu22 environment

[shinrich]

I'm curious why the stderr error output changes so dramatically. E.g., why a purge return in one but not the other.