Skip some tests if zope.app.testing not working.

With Python3 and missing working zope.app.testing we can (and should)
run other tests, but we can skip those tests that are clearly geared
towards accessibility of a working zope.app.testing. These are mainly
the functional tests.

Most tests still fail under Python3 and will have to be separated in
functional and not-functional tests.

src/z3c/testsetup/tests/test_testsetup.py

@@ -3,14 +3,14 @@
 import re
 import sys
 import gc
+import warnings
 import unittest
 from zope.testing import cleanup, renormalizing
 import zope.component.eventtesting
-from z3c.testsetup.util import get_package
+from z3c.testsetup.util import get_package, got_working_zope_app_testing
 
 
 TESTFILES = ['basicsetup.txt',
-             os.path.join('functional', 'functionaldoctestsetup.txt'),
              'pythontestsetup.txt',
              'unitdoctestsetup.txt',
              'util.txt',
@@ -20,6 +20,12 @@
              ]
 
 
+FUNCTIONAL_TESTFILES = [
+    # These tests make only sense with a working zope.app.testing installed
+    os.path.join('functional', 'functionaldoctestsetup.txt'),
+    ]
+
+
 checker = renormalizing.RENormalizing([
     # Relevant normalizers from zope.testing.testrunner.tests:
     (re.compile(r'(\d minutes )?\d+[.]\d\d\d seconds'), 'N.NNN seconds'),
@@ -187,6 +193,15 @@ def test_suite():
     suite = unittest.TestSuite()
     for name in TESTFILES:
         suite.addTest(suiteFromFile(name))
+    if got_working_zope_app_testing():
+        for name in FUNCTIONAL_TESTFILES:
+            suite.addTest(suiteFromFile(name))
+    else:
+        message = (
+            "Skipped the following tests due to missing "
+            "usable `zope.app.testing` package: %s" % FUNCTIONAL_TESTFILES
+            )
+        warnings.warn(message)
     suite.addTest(testrunner_suite())
     suite.addTest(zopeapptestingless_suite())
     return suite