Tests and benchmarks are not runnable from project root

[phadej]

E.g.

cabal run cborg-tests

fails because few data files not found.

Similarly

cabal run versus

fails with

versus: Uncaught exception ghc-internal:GHC.Internal.IO.Exception.IOException:

bench/data: createDirectory: does not exist (No such file or directory)

(Also I think it's wrong to pollute source tree with "temporary" - and not very small - data files).

[dcoutts]

It's awkward since the tests when run via cabal test are run with the CWD as the package root. But cabal run runs using the current CWD, which is often the project root. So we can't just fix it to use project relative paths since then cabal test will break.