[Cleanup Tests] renamed discret tests, moved tpfa nonconforming and the…

[Cleanup Tests] renamed discret tests, moved tpfa nonconforming and the staggered tests in separate folders

• Are the names of the new subfolders OK with everyone?

• box

• cellcentered, with

1. cellcentered/tpfa
2. cellcentered/tpfanonconforming

• staggered, with

1. staggered/freeflow
2. staggered/fv