Clean up files after doing make test

[ejtovar]

When running the Proteus tests via make test, there are a lot of mesh (and other) files that are created during this process. All these files clutter the Proteus directory in my opinion. It would be nice if we could clean up these files after the tests have been completed. There are two options we could do:

1. Have each specific test file clean up its own created files
2. Add a make clean option to the Makefile that removes the files in one fell swoop

What do you think @zhang-alvin @cekees?