Add more notes about test isolation problems.

tests.cfg

@@ -309,10 +309,14 @@ unit_tests =
     z3c.relationfield
 Dexterity =
     plone.app.textfield
+# Moving plone.app.z3cform into the plone_app_testing group leads to deadlocks / very long
+# running robot tests (> 25 min) on Jenkins.
     plone.app.z3cform
 # Moving plone.formwidget.namedfile into the plone_app_testing group leads to test failures
 # in multiple other test groups with:
 # ImportError: No module named iterate
+# Here is the fun fact: those errors do not occur on our new rackspace machines
+# running Ubuntu 14.04.
     plone.formwidget.namedfile
 # Moving plone.folder into the plone_app_testing group leads to test failures with:
 # ValueError: undefined property 'content_meta_type'
@@ -323,6 +327,7 @@ Dexterity =
 # Moving plone.schemaeditor into the plone_app_testing_group leads to test failures.
     plone.schemaeditor
 
+
 # TEST ISOLATION PROBLEMS
 # -----------------------
 #