tests: skip demandimport if disabled

demandimport and setuptools and decorator (from ironpython) and
pygments leads to lots of fail.

If demandimport is disabled we should skip testing it...

tests/test-demandimport.py

@@ -4,6 +4,14 b' from mercurial import demandimport'
 demandimport.enable()
 
 import os
+import subprocess
+import sys
+
+# Only run if demandimport is allowed
+if subprocess.call(['python', '%s/hghave' % os.environ['TESTDIR'],
+                    'demandimport']):
+    sys.exit(80)
+
 if os.name != 'nt':
     try:
         import distutils.msvc9compiler