remove unnecessary dependencies and support spec_ prefix

Created by: minad

Hi,

I removed the unnecessary dependencies from tasty-discover. tasty-hunit, etc are imported if needed. Test writers should also not include Test.Tasty.Discover but the other tasty modules instead - test-discover only acts as a drop-in to aggregate tests.

Additional I added support for the prefix "spec_" (see BarTest from example) which allows to write hspecs directly without falling back on testCase. The code generation doesn't rely on th anymore.

Some of the code is not yet in the best shape (e.g. Run.hs). Please let me know if you are interested in the PR.

Daniel