testsuite: Use Cabal's new test suite interface

* The old way: a user hook in Setup.hs
  * The new way: specify it in gf.cabal
  * The test suite is now called gf-tests, and it runs testsuite/run.hs.
  * You can run it manually with 'runhaskell testsuite/run.hs'. It also runs,
    together with rgl-tests, when you do 'cabal test'
  * Currently only 9 of 34 tests pass. Many failures have silly causes:
    - Error messages that look slightly different
    - Same output but in a different order
    - Absolute paths in output

gf.cabal

@@ -236,3 +236,9 @@ test-suite rgl-tests
   main-is:        run.hs
   hs-source-dirs: lib/tests/
   build-depends:  base, HTF, process, HUnit, filepath, directory
+
+test-suite gf-tests
+  type:           exitcode-stdio-1.0
+  main-is:        run.hs
+  hs-source-dirs: testsuite
+  build-depends:  base>=4.2 && <5, Cabal>=1.8, directory, filepath, process