Also print more informative test failure messages.

Contributed by Stephen Blackheath.

-package-db only exists on GHC >=7.5 so we use (the now deprecated)
 -package-conf flag instead.

GHC and ghc-pkg package db flags changed from '*-package-conf' to
'*-package-db' in 7.5.  This commit follows the change and introduces a
version check whenever those flags are used.

Makes things a little simpler in GHC's build system if libraries are in
the same directory as their name.

At the top level we now have cabal/ and cabal-install/

In case we've got a compiled Setup binary

The purpose of these tests is mostly to pin down some existing behaviour to
ensure it doesn't get broken by the ticket #89 changes.