Test "NOTIFY" message is sent to Nameservers

New test case: "test_notify_msg_sent_to_nameservers"
Test scenario:
1) Create a zone and wait till ACTIVE
2) Update zone's TTL, esure that ACTIVE + Serial has changed
3) Sampling Nameservers up until new Serial is detected in zone's SOA
   record or tiemout has reached.
Note: also includes minor code improvements for existing tests.

Change-Id: I11ed5896902aa869562e797e25444d5052f2fc42
1 file changed