Move the Plan B legacy stats tests to their own file

The previous change gathered the StatsCollectorTrackPlanBTest fixture
and its tests at the end of legacy_stats_collector_unittest.cc. Move
that section verbatim to legacy_stats_collector_plan_b_unittest.cc, so
that the whole file can be deleted when Plan B is removed.

Besides the moved code, the new file only has a file header, a TODO to
delete it, includes and the namespace. Its test target is part of
peerconnection_unittests.

Bug: webrtc:470300031
Change-Id: I3f84d327829d00fa0f470c2459f962a52428d839
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/508429
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48909}