Harmonize source, input and executable file names in test folder

Following the majority, I'd suggest test_model_problem/appl specific.cc/input for the source/input and test_model_problem/appl specific_discretization for the target. The problem/appl specific for the source/input may be shortened if it appears already as a subfolder name.

For example, in test/porousmediumflow/2p/implicit/incompressible, the source/input would be named test_2p.cc/input, while a target would be named test_2p_incompressible_tpfa. I could also live with only main.cc for the source if there only is one, but wouldn't know something good for the input.

I would omit the fv since all we do is fv and I would not expect the source/input to become different once we use fe or fd.

For the multidomain models, I'd put an underscore between the coupled models, so test_stokes1p2c_darcy1p2c_horizontal rather than test_stokes1p2cdarcy1p2chorizontal.

Opinions?

Has been implemented in !1284 (merged)