Improve test_set_members_deleting_less_than_5

Improve test_set_members_deleting_less_than_5

In test_set_members_deleting_less_than_5[1], 3 ips are deleted from
ipset but test_set_members_deleting_less_than_5 checked that the
first one was deleted because the call ordering was non-trivial.

The test was successful because
assert_has_calls(expected_calls, any_order=False) allows extra calls
before and after expected_calls.

A parent change[2] forces the call ordering, this allows to check that
the 3 ips are deleted.

[1] neutron.tests.unit.agent.linux.test_ipset_manager
[2] I8408365825ec1e97a83c2181f38ec1f9468df91e

Related-Bug: #1348818

Change-Id: I773e2cd19fdec634b728d0c3a78c1d66392c743f