Making peerconnection_unittest use the virtual socket server it creates.
Making peerconnection_unittest use the virtual socket server it creates.

When this test was updated to use separate worker/signaling threads,
the virtual socket server wasn't moved over to the worker thread. So it
was set on the signaling thread and wasn't being used.

Review-Url: https://codereview.webrtc.org/2015763002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#12951}

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