Add tests for benchmarks and hspec-discover

[rvl]

This adds test cases for two bugs:

1. benchmarks don't build (fixed in builder: Install benchmarks properly #175)

2. If the cabal package has a test-suite with build-tools: hspec-discover, then cabal will not work in the development shell:

   # cabal new-configure --enable-tests
   Resolving dependencies...
   cabal: Could not resolve dependencies:
   [__0] trying: project-0.1.0.0 (user goal)
   [__1] rejecting: project:!test (constraint from config file, command line
   flag, or user target requires opposite flag selection)
   [__1] trying: project:*test
   [__2] next goal: project:hspec-discover:exe.hspec-discover (dependency of
   project *test)
   [__2] rejecting:
   project:hspec-discover:exe.hspec-discover-2.7.1/installed-EHO... (does not
   contain executable 'hspec-discover', which is required by project *test)
   [__2] fail (backjumping, conflict set: project,
   project:hspec-discover:exe.hspec-discover, project:test)
   After searching the rest of the dependency tree exhaustively, these were the
   goals I've had most trouble fulfilling: project, project:test,
   project:hspec-discover:exe.hspec-discover   
   

   The tests component builds fine, because hspec-discover is provided in the PATH.

   But I think the build-tools which are haskell packages need to be added into the package database for shells.

[angerman]

@rvl I somehow completely missed that. Can we rebase it and merge it? Should we disable the tests?

[rvl]

This PR adds regression tests for two bugs. I fixed one of them, but haven't figured out the other one yet.

[angerman]

I'm really tempted to merge this @rvl, even if that means we have a broken test? Can we disable that for now with a comment?

[rvl]

OK if you want to merge I have removed the attempted fix and opened an issue #231.