As the error message suggests, the tests still require a graphics set to run (as they're actually just running the game). OpenGFX works well (which is what we use in the CI), or if you're really interested in minimalism, you can find a "nogfx" variant somewhere around the internet

Oh, and despite it needing the "graphics", it doesn't actually render anything, so xvfb is unnecessary

I assume the above helped you out. If not, here some more snippets:

https://github.com/OpenTTD/OpenTTD/blob/master/.github/workflows/ci-build.yml#L113

This is what we do to get OpenGFX running, and making a graphics set available for the regression test.

Feel free to reopen if that doesn't fix your issue!

I assume the above helped you out. If not, here some more snippets:

Issue is that all my build systems are intentionally cut off from public network.
Is it possible to point to system installed OpenGFX?

Any way you can feed OpenGFX on the system will work fine. As long as the game can find it in any search-path, it will pick up on it. So if you have another way to get it in ~/.local/share/openttd/baseset for example, it will work just fine! (or the global variant ofc, but I always forget the exact path .. /usr/share/.. something :D)

I've been trying to run test suite over xv-fb and it does not work

+ cd OpenTTD-12.2
+ xvfb-run -a /usr/bin/make -O -j48 V=1 VERBOSE=1 -C x86_64-redhat-linux-gnu test ARGS=--output-on-failure
make: Entering directory '/home/tkloczko/rpmbuild/BUILD/OpenTTD-12.2/x86_64-redhat-linux-gnu'
Running tests...
/usr/bin/ctest --force-new-ctest-process --output-on-failure
Test project /home/tkloczko/rpmbuild/BUILD/OpenTTD-12.2/x86_64-redhat-linux-gnu
    Start 1: regression_regression
1/2 Test #1: regression_regression ............***Failed    0.10 sec
1: -
1: + Error: Failed to find a graphics set. Please acquire a graphics set for OpenTTD. See section 1.4 of README.md.'
2: - --TestInit--
2: + '
(9539 lines were expected but 2 were found)
CMake Error at /home/tkloczko/rpmbuild/BUILD/OpenTTD-12.2/cmake/scripts/Regression.cmake:105 (message):
  Regression failed - Output in
  /home/tkloczko/rpmbuild/BUILD/OpenTTD-12.2/x86_64-redhat-linux-gnu/regression_regression_output.txt



    Start 2: regression_stationlist
2/2 Test #2: regression_stationlist ...........***Failed    0.08 sec
1: -
1: + Error: Failed to find a graphics set. Please acquire a graphics set for OpenTTD. See section 1.4 of README.md.'
2: - --StationList--
2: + '
(128 lines were expected but 2 were found)
CMake Error at /home/tkloczko/rpmbuild/BUILD/OpenTTD-12.2/cmake/scripts/Regression.cmake:105 (message):
  Regression failed - Output in
  /home/tkloczko/rpmbuild/BUILD/OpenTTD-12.2/x86_64-redhat-linux-gnu/regression_stationlist_output.txt




0% tests passed, 2 tests failed out of 2

Total Test time (real) =   0.18 sec

The following tests FAILED:
          1 - regression_regression (Failed)
          2 - regression_stationlist (Failed)
Errors while running CTest

Error: Failed to find a graphics set. Please acquire a graphics set for OpenTTD. See section 1.4 of README.md.

Install OpenGFX in the test suite environment, then retry the text suite.

Additionally, the test suite runs using the 'null' video driver and does not attempt to actually open any windows or draw any graphics. It does not need X11 or Wayland or other graphical display system available, but the game logic requires a graphics set to be loaded.