[roll] Roll fuchsia [testrunner] Resolve test packages in the background

Modifies testrunner to (optionally) serially resolve test packages in a
background goroutine to hide some of the latencies associated with ephemeral
package delivery.

Run-All-Tests: True

Original-Bug: b/226144399
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/697124
Original-Revision: 276514f0b77b2d874ea1815351916bec090e6bf3
GitOrigin-RevId: cb3d99309ada596bd6ef225952142f81b0999a1c
Change-Id: I0cd102064f6a27690e2e9fc25b56a7a90b14bdde
diff --git a/stem b/stem
index e1a603d..de14830 100644
--- a/stem
+++ b/stem
@@ -10,7 +10,7 @@
              githooks=""
              remote="https://fuchsia.googlesource.com/fuchsia"
              gerrithost="https://fuchsia-review.googlesource.com"
-             revision="6870e6afc3456870abd9e79ad8f2609fb939b36c"/>
+             revision="276514f0b77b2d874ea1815351916bec090e6bf3"/>
   </projects>
   <hooks>
     <hook name="install-environment"