[roll] Roll fuchsia [bt][rfcomm] Add SessionMultiplexer unit tests

Add basic unit tests to verify the starting of the multiplexer &
establishing a user DLCI.

Original-Fixed: 42140263
Test: fx test bt-rfcomm-tests
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1192492
Original-Revision: 7d1500aaabae255ccc58cce35bb6e11a0106a3a9
GitOrigin-RevId: f3e010e3d82edaa18d1ec3eee96e88624b4f2668
Change-Id: I1df54c4bc3f40613be991f65e0580e5215557d22
diff --git a/stem b/stem
index 1fc3422..741f24c 100644
--- a/stem
+++ b/stem
@@ -11,7 +11,7 @@
              githooks="integration/git-hooks"
              remote="https://fuchsia.googlesource.com/fuchsia"
              gerrithost="https://fuchsia-review.googlesource.com"
-             revision="1037ef20df897148fef41391db30e84c53efa12e"/>
+             revision="7d1500aaabae255ccc58cce35bb6e11a0106a3a9"/>
   </projects>
   <hooks>
     <hook name="install-environment"