The problem may be related to the fact that the line
include(${Slicer_EXTENSION_GENERATE_CONFIG})
was not in the CMakeLists file. I'll report back when I find out if it improved things.

It didn't do the trick, but I'll keep trying.

The issue is that SlicerRT extension generates its SlicerRTConfig file in the top level directory but the extension build system expects the file in the inner build directory.

Inseed, it sets SlicerRT_DIR appending the inner build directory.

See https://discourse.slicer.org/t/nightly-build-tests-failing-for-extensions-with-dependencies-to-other-scriptable-modules/1483/18

The test finds the dependencies and starts as expected, so I'm closing the issue.

I have one question though. There is an error in the test (which I'll fix), but the test passes. How can I make it fail if an exception happens? I can make any calls when handling the exception. Thanks!
http://slicer.cdash.org/testDetails.php?test=8440601&build=1134892