extract Properties module for re-use by contrib tests

I want to reuse Properties' Arbitrary instance (as well as the T and
NonNegative types) in an upcoming set of SwapWorkspaces QC props.
`module Main where import Main` doesn't work too well. :)

If this patch is accepted, the darcs 'test' pref should be modified to
"-itests tests/Main.hs".

tests/Main.hs

@@ -0,0 +1,8 @@
+module Main where
+
+import qualified Properties
+
+-- This will run all of the QC files for xmonad core. Currently, that's just
+-- Properties. If any more get added, sequence the main actions together.
+main = do
+  Properties.main

tests/Properties.hs

@@ -1,4 +1,5 @@
 {-# OPTIONS -fglasgow-exts #-}
+module Properties where
 
 import StackSet hiding (filter)
 import qualified StackSet as S (filter)