[roll] Roll fuchsia [run-test-suite] stdout reporter part 3: Reporter implementation

This is part 3 of a refactor to move stdout output into a reporter
implementation. This change adds the actual stdout reporter
implementation, and the multiplexer reporter needed to support
reporting to both the directory and stdout reporters.
This change doesn't yet use the reporter.

Testing: new unit tests

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/601526
Original-Revision: 392c43726f496bced75cfb63fd835eb9608000c6
GitOrigin-RevId: e8b4215ebf260d67ee0a9c6ba735ea9d51bacc97
Change-Id: I4108927808596fcc4162248e86dbae185d5fc81e
diff --git a/stem b/stem
index 37700ce..6155334 100644
--- a/stem
+++ b/stem
@@ -10,7 +10,7 @@
              githooks=""
              remote="https://fuchsia.googlesource.com/fuchsia"
              gerrithost="https://fuchsia-review.googlesource.com"
-             revision="ac59a683a2e921818937fe288992165a271cc6ca"/>
+             revision="392c43726f496bced75cfb63fd835eb9608000c6"/>
   </projects>
   <hooks>
     <hook name="install-environment"