Allow SDP munging in peerconnection_unittests

Configure peerconnection_unittests to allow specific munging types
(such as kVideoCodecsFmtp, kAudioCodecsRemoved, etc.) under the
WebRTC-NoSdpMangleAllowForTesting field trial, as these tests
explicitly perform SDP munging to verify behavior.

Specifically allow CCFB munging (kAudioCodecsRtcpFb and
kVideoCodecsRtcpFb) in tests that negotiate CCFB and use
PeerConnectionTestWrapper, due to CCFB negotiation asymmetry.

Refactored CreatePeerConnection helpers in tests to use overloading
instead of default arguments.

Bug: webrtc:414284082
Change-Id: I2cb72fce127dcde8e334408e3a704d8b618058d5
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/482360
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48043}