Switch Haskell test harness to test-framework

This patch replaces our home-grown, and quite limited, test runner
infrastructure with test-framework
(http://batterseapower.github.com/test-framework/

). The rationale for
doing so is as follows:

- we will need to add support for HUnit tests, so either we add more
  custom code or we switch to an existing library
- test-framework is mature and already packaged, at least in
  Debian/Ubuntu
- it supports more features: parallel test running, better test
  selection, etc.

As you can see, the changes are trivial, and don't touch the tests at
all; if/when we split the QC.hs file into per-module files, then we
could drop QCHelper too, and replace it with test-framework-th, which
does the same, but even more automated (auto-discovery, without having
to list the tests at all).

Dependencies are updated in devnotes.rst; note that I've already added
the hunit dependencies since we're going to use that soon.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>