Add a doctest that works without z.a.testing.

This is a stripped-down version of setupininit.txt. It requires that
no working zope.app.testing is installed.

Why another doctest? Turned out, that turning this doctest
('setupininit.txt') into a regular Python test, would be even more
complex. Also conditional doctest parts (i.e.: parts of a doctest file
that are only run if a certain condition is met) seem not to be
possible. At least I found no (easily readable) way to write something
like:

  >>> if cond:
  ...   run_one_test()
  result of run_one_test

  >>> if not cond:
  ...   run_other_test()
  result of run_other_test.

where one of the results would be ignored. Any hints would be welcome.

src/z3c/testsetup/tests/setupininit_nonfunctional.txt

@@ -0,0 +1,43 @@
+Setting up tests in packages __init__.py
+----------------------------------------
+
+.. warning:: The following works only *without* working zope.app.testing!
+    It is a stripped-down version of `setupininit.txt`.
+
+We can also setup tests in packages' __init__ modules. We defined such
+a test setup in the cave package::
+
+    >>> import os
+    >>> import z3c.testsetup
+    >>> pkgpath = os.path.dirname(z3c.testsetup.__file__)
+    >>> cavepath = os.path.join(pkgpath, 'tests', 'cave')
+
+The setup looks like this::
+
+    >>> setupfile = os.path.join(cavepath, 'tests', '__init__.py')
+    >>> print(open(setupfile).read())
+    # this is a package that contains a testsetup.
+    #
+    # To let it be found by the testrunner, you must call the testrunner
+    # with the approriate options set.
+    import z3c.testsetup
+    test_suite = z3c.testsetup.register_all_tests('z3c.testsetup.tests.cave')
+
+Now we run this test, requiring `__init__` as the test-file-pattern
+for the testrunner::
+
+    >>> import sys
+    >>> defaults = [
+    ...     '--path', cavepath,
+    ...     '--tests-pattern', '^tests$',
+    ...     '--test-file-pattern', '__init__',
+    ...     ]
+    >>> sys.argv = [sys.argv[0]]
+    >>> from z3c.testsetup import testrunner
+    >>> testrunner.run(defaults)
+    Running zope.testrunner.layer.UnitTests tests:
+      Set up zope.testrunner.layer.UnitTests in 0.000 seconds.
+      Ran 2 tests with 0 failures and 0 errors in 0.003 seconds.
+    Tearing down left over layers:
+      Tear down zope.testrunner.layer.UnitTests in 0.000 seconds.
+    False