[xmonad] QuickCheck properties for layouts

[Don Stewart]

dons:
> I just pushed a patch fully testing, via QuickCheck properties, the
> builtin tall layout, and how it handles messages. That code is fully
> covered now.
> 
> To layout authors, using pureLayout and pureMessage, have a poke around
> Properties.hs , to get an idea how to generate random window manager
> states, apply your layouts to them, and then to test their behaviour
> with random message events.

Oh, also, there's a built in script now for getting code coverage
details from the QuickCheck suite (see tests/coverage.hs).

Note, these are the parts of the code tested on every commit -- you
can't commit unless the entire suite runs to success.

Results here:

    http://galois.com/~dons/tmp/hpc_index.html

StackSet.hs, where most of the logic is, is fully covered,
Layout.hs starting. Should help drive testing.

-- Don