run-tests: expand user in --with-hg stable

run-tests: expand user in --with-hg

This makes it possible to run:
  $ ./run-tests.py --with-hg=~/hg-bin/hg

--- a/tests/run-tests.py	Fri Jan 20 01:24:16 2012 +0100
+++ b/tests/run-tests.py	Sat Jan 14 01:56:27 2012 +0100
@@ -200,6 +200,7 @@
         options.pure = True
 
     if options.with_hg:
+        options.with_hg = os.path.expanduser(options.with_hg)
         if not (os.path.isfile(options.with_hg) and
                 os.access(options.with_hg, os.X_OK)):
             parser.error('--with-hg must specify an executable hg script')