[!127] Resolve "Support system tests with pseudo CMake targets"

Merge branch '135-support-system-tests-with-pseudo-cmake-targets' into 'master'

ref:quality/dune-testtools This MR resolves #135 by not passing the executable key and argument (--exec <arg>) to the python script if the passed target is not an executable. This way, executables in other directories will keep working as well as for custom targets with custom wrappers.

Closes #135

See merge request quality/dune-testtools!127

Closes #135 (closed)

(cherry picked from commit dd51fb25)

ab6d4da1 Check wheter a target is an executable before path expansion 9341bf68 [python][scripts] Allow keys to be either in the vtk subsection or in the... 60a82248 Revert "[python][scripts] Allow keys to be either in the vtk subsection or in...