Pythonic method names for l3_agent unit tests

When review https://review.openstack.org/#/c/46863/, i find there
are some unconsistent method names in test_l3_agent.py, this patch
fixes this code style problem.

Change-Id: I0e657e02155e1fcd580f083f4b5ba56274eadf88
Partial-Bug: #1229155
Merge "Pythonic method names for l3_agent unit tests"