[Unit Tests] test_SolverParams fails on Appveyor

test_solverprams fails on appveyor because Qt platform plugin "windows" seems to be lacking.

Output log :

 27/112 Test  #27: test_solverparams ........................................................***Failed    0.08 sec
[==========] Running 17 tests from 1 test case.
[----------] Global test environment set-up.
[----------] 17 tests from test_SolverParams
[ RUN      ] test_SolverParams.load_non_existing_data_model_file
This application failed to start because it could not find or load the Qt platform plugin "windows"
in "".
Reinstalling the application may fix this problem.