Add unit test for NTP offset estimation using RRTR/DLRR packets

This CL adds a new test case 'EstimateWithRrtrAndDlrrRtt' to
RemoteNtpTimeEstimatorTest. The test verifies that the NTP clock offset
is correctly estimated when the Round-Trip Time (RTT) is derived from
RTCP XR RRTR (Receiver Reference Time Report) and DLRR (Delay since
Last Receiver Report) blocks.

The test simulates:
1. Receiver sending RRTR with local NTP time T1.
2. Remote sender responding with DLRR containing T1 and a processing delay.
3. Receiver calculating RTT from DLRR arrival time and delay fields.
4. Estimator updating NTP offset using this RTT and multiple Sender Reports.

Bug: webrtc:41480926
Change-Id: Ia4335a93801f02cdab8771f347d4c600bb66bf77
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/444201
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46777}