Hi, and thank you for making me feel welcome! I have already gotten help from Ron, the author of gr-iqlevels. Though unfortunately, due to not being able to open mailto links with with the Outlook web app, I responded to him with a different email address,
and therefore our discussion has not been included in the mailing list correctly. Hopefully I am responding in a correct way now. Below I will outline the solution for posterity.
Ron suggested using cmake -DCMAKE_INSTALL_PREFIX=/usr ../ with the motivation "Since you're using the ppa, you installed in the wrong directory.". So for gr-iqlevels the following sequence gets us past the error:
git clone email@example.com:drmpeg/gr-iqlevels.git
cmake -DCMAKE_INSTALL_PREFIX=/usr ../
sudo make install
Now, the test will still fail because the QA class have actually not been implemented (its just the template generated by gr_modtool). But if line 30 of gr-iqlevels/python/qa_iqlevels.py is changed to instance = iqlevels(1000000,True) then the test goes
Ron also suggested using sudo ldconfig after sudo make install. This does not seem to make any difference for me though, and I am not sure if ldconfig actually even does anything (it outputs nothing to terminal).
Providing the install prefix to cmake also resolved the corresponding issue with the tutorial howto module, as expected. (I still initially ran into issues with an "Unable to coerce endpoints" error, but this was resolved when I restarted the tutorial
from the beginning, so it was probably due to some silly mistake that I wasn't able to identify.)