client: Add unittest for GetUrlAndWait().

This CL adds unittests for GetUrlAndWait(). In order to simulate
the scenario, further testing infrastructure and unittesting for
the PeerSelector is added.

BUG=chromium:264384
TEST=unittests pass.

Change-Id: I380b8c09e33a2acc53979b841d682e011c1d2d8c
Reviewed-on: https://gerrit.chromium.org/gerrit/63819
Reviewed-by: Chris Sosa <sosa@chromium.org>
Commit-Queue: Alex Deymo <deymo@chromium.org>
Tested-by: Alex Deymo <deymo@chromium.org>
10 files changed