Allow running just non-fuzzed tests

[cllns]

I have a suite of fuzzed and non-fuzzed tests. Sometimes I'd like to be able to run just the non-fuzzed tests, since they're much much faster (and the fuzzing floods the terminal with tons of noisy failures, see: #327)

A simple way to do this would be to allow elm-test --fuzz 0. Right now it says:

error: option '--fuzz <int>' argument '0' is invalid. Expected at least 1.

If this is implemented, there should be a conspicuous notice that all fuzz tests are skipped.

Another option would be calling the CLI arg --skip-fuzz

[mpizenberg]

As a side note, one can put their fuzz tests and normal tests in two different directories, and pass the directory they want as a CLI argument. This may need a bit of code shuffling around but it should solve your issue.