Add helper to determine zope.app.testing usability.

While z3c.testsetup can work also without zope.app.testing
installed, it runs into trouble when z.a.testing is installed but
not in a usable state. This is the case with Python 3.x at the
moment: you can install it, but it will be broken.

Therefore we need a way to determine clearly (in tests and code)
whether we can import *and use* zope.app.testing. The new function
is meant for that purpose.

src/z3c/testsetup/tests/test_util.py

@@ -0,0 +1,17 @@
+""" Tests for `z3c.testsetup.util`.
+
+Most of `z3c.testsetup.util` helper functions are tested in doctest
+files called ``util.txt`` (yes, there are two of them). As this kind of
+doctesting is a hell to debug and (in times of Sphinx) even not very
+useful for documentation, we write down new tests in Python.
+"""
+import unittest
+from z3c.testsetup.util import got_working_zope_app_testing
+
+
+class TestUtil(unittest.TestCase):
+
+    def test_got_working_zope_app_testing(self):
+        # we can generally determine whether zope.app.testing is useable
+        result = got_working_zope_app_testing()
+        assert (result is True) or (result is False)

src/z3c/testsetup/util.py

@@ -115,3 +115,21 @@ def get_attribute(name):
     name, attr = name.rsplit('.', 1)
     obj = import_name(name)
     return getattr(obj, attr)
+
+
+def got_working_zope_app_testing():
+    """Determine whether we have a working `zope.app.testing` installed.
+
+    Please do not make any assumptions about how the "workingness" of
+    `zope.app.testing` in an environment is determined.
+    """
+    try:
+        from zope.app.testing.functional import (
+            HTTPCaller, getRootFolder, sync, ZCMLLayer,
+            FunctionalDocFileSuite, FunctionalTestSetup)
+        return True
+    except:
+        # any problem here (not only ImportError) is an indicator for
+        # serious trouble with zope.app.testing.
+        pass
+    return False