tests/test_cel: Fix CEL unit test failures caused by attended transfer changes

When the publication of attended transfer messages were pushed to another
thread, some subtle race conditions were introduced with the CEL unit tests.
This patch fixes one of them, and pushes the other to ASTERISK-22367, which
already exists to fix another bouncy CEL unit test.

In particular, this patch fixes the test_cel_attended_transfer_bridges_link
test, and defers the test_cel_attended_transfer_bridges_swap test to the
aforementioned JIRA issue.

ASTERISK-22367


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@428891 f38db490-d61c-443f-a65b-d21fe96a405b