Change mpas-run step from a Given to a When

I have changed the semantic style of performing an MPAS run from a
"Given" to a "When".

All features should be changed like this:
- Given A 1 processor MPAS "landice_model_testing" run
- Given A 4 processor MPAS "landice_model_testing" run with restart
+ When I perform a 1 processor MPAS "landice_model_testing" run
+ When I perform a 4 processor MPAS "landice_model_testing" run with restart

I also made the 'A setup test environment' step name and function more
semantically matching the other steps.