Write haskell tests to test things in Main.hs

The command line arguments and the recursive searching for .elm files in directories are currently tested in the `./tests/run-tests.sh` script. Supposedly it is possible to write tests in haskell that test IO. It would be great to get some examples of such haskell tests added so that we can replace the shell script tests for testing all the different command line arguments.

Start with Main.hs and tests/run-tests.sh to understand the current command-line argument and recursive .elm-file search coverage. Identify how Haskell IO tests can exercise those cases, add examples, and confirm the Haskell tests cover the shell script scenarios before replacing them.

Written by the indexing model from the issue text.