Deleting A/B Test mailings causes problems

Creating new A/B-test and deleting the 'test' mailings from 'Scheduled / Sent' mailings list has strange effects. The 'parent' a/b test is not deleted, but is not usable (clicking Continue doesn't work).

We need to make the following changes:
1. block deletion of the "child" mailing test records, and tell users to delete from Manage AB Tests if they want to delete the set.

2.. Add 'delete' action to Manage AB Tests selector, and make sure that works for 'broken' AB Tests (childless). Also give a sensible message on clicking Continue when the AB Test is 'broken'.

— original post —
Example: see drupal-demosite (v4.6.6) -> mailings -> manage a/b tests and try to continue test with date 2015-07-17 15:19:51 -> doesn't work
(http://d46.demo.civicrm.org/civicrm/a/#/abtest/1)

in my installation (v4.6.4), there are also strange interferences with other mailings created after deleting the a/b-test. continue of such a mailing opens the link "../civicrm/a/#/abtest/1" instead of e.g. "../civicrm/a/#/mailing/46"