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Add tests which exercise wifi_sync's ability to add networks.
These tests add a network to the platform wifi config, and then
verify that the new network appears in other sync clients.

BUG=chromium:431435
TEST=sync_integration_tests --gtest_filter="*Wifi*"

Review URL: https://codereview.chromium.org/843483004

Cr-Commit-Position: refs/heads/master@{#315070}