In searching for resources on the missing libCTKQtTesting.so library, in found:
An sure enough, libCTKQtTesting.so was merely just the first shared lib not found:
~/t/S/S/Slicer-build> ./Slicer --launch ldd ./bin/SlicerApp-real | head
linux-vdso.so.1 (0x0000773d3e11d000)
libqSlicerApp.so => /home/ruffsl/tmp/Slicer-5.9/Slicer-build/Slicer-build/bin/./libqSlicerApp.so (0x0000773d3e073000)
libqSlicerBaseQTApp.so => /home/ruffsl/tmp/Slicer-5.9/Slicer-build/Slicer-build/bin/./libqSlicerBaseQTApp.so (0x0000773d3e01f000)
libqSlicerModulesCore.so => /home/ruffsl/tmp/Slicer-5.9/Slicer-build/Slicer-build/bin/./libqSlicerModulesCore.so (0x0000773d3dfff000)
libqSlicerBaseQTCLI.so => /home/ruffsl/tmp/Slicer-5.9/Slicer-build/Slicer-build/bin/./libqSlicerBaseQTCLI.so (0x0000773d3df9d000)
libqSlicerBaseQTGUI.so => /home/ruffsl/tmp/Slicer-5.9/Slicer-build/Slicer-build/bin/./libqSlicerBaseQTGUI.so (0x0000773d3dc00000)
libqMRMLWidgets.so => /home/ruffsl/tmp/Slicer-5.9/Slicer-build/Slicer-build/bin/./libqMRMLWidgets.so (0x0000773d3d800000)
libCTKQtTesting.so.0.1 => not found
libCTKVisualizationVTKWidgets.so.0.1 => not found
libCTKScriptingPythonWidgets.so.0.1 => not found
After downloading and inspecting the buildfarm artifacts for the same commit, noticed the file tree was different compared to my local cmake build folder, and that I was attempting to run the Slicer binary directly, rather than from the extracted launcher binary from the generated .tar.gz archive nested inside the very same local build.
Sure enough, the packaged version is working as expected without any missing libs:
~/t/S/S/Slicer-build/Slicer-5.9.0-2025-08-25-linux-amd64$ ./Slicer \
--launch ldd ./bin/SlicerApp-real | grep "not found"
~/t/S/S/Slicer-build/Slicer-5.9.0-2025-08-25-linux-amd64$
Is this the Cmake macro triggered by this corresponding command? Cpack is new to me.
$slicer_build_env_script cmake --build Slicer-build/Slicer-build --target package