* reorganize travis matrix, extend conftest.py

* pytestify bdalg_test

* pytestify canonical_test

* pytestify config_test

* pytestify convert_test.py

* pytestify ctrlutil_test

* pytestify delay_test.py

* pytestify discrete_test

* pytestify flatsys_test

* pytestify frd_test

* pytestify freqresp_test.py

* pytestify input_element_int_test

* pytestify iosys_test

* pytestify lti_test.py

* pytestify mateqn_test

* pytestify matlab tests

* pytestify minreal_test

* pytestify modelsimp

* pytestify nichols_test

* pytestify phaseplot_test

* pytestify rlocus_test.py

* pytestify robust tests

* pytestify sisotool_test.py

* pytestify slycot_convert_test.py

* pytestify statefbk tests

* pytestify statesp tests

* pytestify timeresp_test.py

* pytestify xferfcn_input_test.py

remove a lot of duplicate code by converting everything into
a single parametrized test function.

* pytestify xferfcn tests

* make tests work with pre #431 source code state

revert this commit when merging a rebased #431

(remove statesp_test.py::test_copy_constructor_nodt if not applicable)