Avoid race in tests

[bmwiedemann]

When building openSUSE's python-xcffib package
without this patch on a 1-core VM on a busy host,
tests failed with

 FAIL: test_python_code.TestXcffibTestGenerator.test_XcffibTest_generator
 ----------------------------------------------------------------------
 Traceback (most recent call last):
   File "/usr/lib/python2.7/site-packages/nose/case.py", line 197, in runTest
     self.test(*self.arg)
   File "/home/abuild/rpmbuild/BUILD/xcffib-0.9.0/test/test_python_code.py", lin
e 154, in test_XcffibTest_generator
     assert os.environ['DISPLAY'] != old_display
 AssertionError

This PR was done while working on reproducible builds for openSUSE.

[tych0]

This doesn't really avoid the race, just makes it less likely, I think. What are the values of old_display and os.environ['DISPLAY'] when it dies? (nosetests -v should tell you this)

[bmwiedemann]

somehow I cannot reproduce the issue anymore.

given the assertion, I'd think,
old_display and os.environ['DISPLAY'] would have had the same values.

[tych0]

Naturally :). I'm curious what they were. I'll go ahead and close this for now, if we can come up with a better fix for the race than a sleep, I'm open to it. Thanks!