Add Test-Suite so it can be ran with `cabal test`

We build the GitYrp library so we can define the GitYrps
dependencies there once and then depend just on GitYrp when making
executables or testsuites.

And that requires to split the src/ further into library dir and exe/ as
if it is in one dir, the executable gets built against the library files
inside the directory, instead of against a built library which would
again need to specify built-depends and defeats the purpose from
previous paragraph.