Add tests for SSL role and datachannel ID assignment.

Document with a comment the suspected place that could cause a bug.
Also fix an error in previous role observation code.

Bug: webrtc:13668
Change-Id: Id7f6af6905d90f7974b5570145c201c8339aaf72
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/251388
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35973}
diff --git a/pc/dtls_transport.cc b/pc/dtls_transport.cc
index 5cee54c..e8d6ae9 100644
--- a/pc/dtls_transport.cc
+++ b/pc/dtls_transport.cc
@@ -114,10 +114,10 @@
       if (success) {
         switch (internal_role) {
           case rtc::SSL_CLIENT:
-            role = DtlsTransportTlsRole::kServer;
+            role = DtlsTransportTlsRole::kClient;
             break;
           case rtc::SSL_SERVER:
-            role = DtlsTransportTlsRole::kClient;
+            role = DtlsTransportTlsRole::kServer;
             break;
         }
       }