Test suite doesn't build on darwin

[sternenseemann]

Due to the case insensitive nature of darwin's file system GHC will complain that it can't distinguish between hedgehog.hs which is supposedly a Main module and the Hedgehog module in scope, resulting in the following, quite confusing error:

test/Hedgehog.hs:1:1: error:
    File name does not match module name:
    Saw: ‘Main’
    Expected: ‘Hedgehog’

Renaming the test entrypoint to Main.hs or similar should fix this.