p2p_transport_channel_unittest : put frequently used checks into functions.

this patch is puts frequently used check into a set of Check-functions.

the behavior of p2p_transport_channel_unittest is almost unchanged,
the minor change is that when waiting for connection between specific
addresses it waits and does not assume that a particular set of
local/remote addresses will be selected first.

the patch also changes a few EXPECT_ to ASSERT_ since the
tests are not useful where the first EXPECT fails.

BUG=webrtc:10647

Change-Id: Iddcc3c88114db80576e9ebc500572a00dbafdd84
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/159882
Commit-Queue: Jonas Oreland <jonaso@webrtc.org>
Reviewed-by: Honghai Zhang <honghaiz@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29812}