pythonPackages.pypeg2: fix nondeterministic test failure

If test_xmlast runs before test_pyPEG2, the tests fail with:

ERROR: runTest (pypeg2.test.test_pyPEG2.ParseInvisibleTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/build/pyPEG2-2.15.2/pypeg2/test/test_pyPEG2.py", line 258, in runTest
    self.assertEqual(r._ignore1, None)
AttributeError: 'C1' object has no attribute '_ignore1'

https://hydra.nixos.org/build/74632015

pkgs/top-level/python-packages.nix

@@ -16941,6 +16941,11 @@ EOF
       sha256 = "0v8ziaam2r637v94ra4dbjw6jzxz99gs5x4i585kgag1v204yb9b";
     };
 
+    checkPhase = ''
+      # The tests assume that test_xmlast does not run before test_pyPEG2.
+      python -m unittest pypeg2.test.test_pyPEG2 pypeg2.test.test_xmlast
+    '';
+
     #https://bitbucket.org/fdik/pypeg/issues/36/test-failures-on-py35
     doCheck = !isPy3k;