Namespace: Fix tests that didn't wrap properly

Insert "namespace webrtc" and an anonymous namespace around
tests that did not have it, and delete superfluous "using webrtc::"
and "webrtc::" prefixes.

Bug: webrtc:42232595
Change-Id: Icd35b2e5c73abe495c387dcae5f7e1f5d785ce92
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/477660
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Auto-Submit: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47866}