Execute old tests only if z.a.testing is avail.

The tests of long time deprecated functionality are not worth being
run with no zope.app.testing available. I checked that no Python3
related errors will occur if the test is enabled under py3.

src/z3c/testsetup/tests/test_testsetup.py

@@ -15,7 +15,6 @@
     'util.txt',
     'unittestsetup.txt',
     os.path.join('tests', 'util.txt'),
-    'tests/README_OLD.txt',
     'testgetter.txt',
     'testrunner.txt',
     'README.txt',
@@ -32,6 +31,7 @@
     # These tests make only sense _with_ a working zope.app.testing installed
     os.path.join('tests', 'setupininit.txt'),
     os.path.join('functional', 'functionaldoctestsetup.txt'),
+    'tests/README_OLD.txt',
     ]