[ Fix #12 ] cabal test-suite to run goldplate's own tests

[andreasabel]

Adds:

• test-suite stanza to the cabal file to run tests/Tests.hs
• tests/Tests.hs simply invokes goldplate via cabal
• run tests on GitHub Actions

[andreasabel]

This PR is ready.
I recommend squashing, since there was some back-and-forth with getting cabal test to run on the Haskell CI action.

[jaspervdj-luminal]

Very nice, thanks!