Add test command

[sol]

Running Hspec tests on CI currently still requires manual steps:

1. The .cabal file has to be modified, changing test-suite to executable so that mcabal builds it.
2. hspec-discover has to be invoked manually because mhs does not interpret GHC options -F -Fpgm.
3. The test suite has to be invoked manually.

This PR addresses (1) and (3) by adding a test command.

showcase

hspec/hspec-example#38

[augustss]

Thanks!
Please, file an issue about -F in the MicroHs repo so I don't forget.