Blender Git Loki

Tests: run tests from install path

Blender can only be run correctly from the install path since it requires Python
scripts, dynamic libraries and other files to be present. By default the install
path is the same as the build path, so it works anyway. But on the buildbot it
isn't. There was a workaround but it failed on Windows and macOS.

Now tests run from the install path. Detecting that path for ctest is more
complicated than I would like, but I couldn't find a better solution.

Ref T69541.