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

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

tests/run-tests.py

@@ -200,6 +200,7 b' def parseargs():'
         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')