Add requests_mock to test-requirements.txt
Add requests_mock to test-requirements.txt

Already in global-requirements. Will use for VPN unit tests
(bug 116018).

Change-Id: I140caebd3bc4f4cbf9563698bf0143f68456c762
Closes-Bug: 1358472

yamt/networking-ofagent,yamt/networking-ofagent-old3,budnyjj/ofagent,budnyjj/ofagent