[Fuchsia] Include fuchsia_web/av_testing:av_sync_tests in the deps

The way ninja works requires the target to be accessible from
the root, so now the builder [1] is failing due to the fact it
cannot find the target.

[1]: https://ci.chromium.org/ui/p/chrome/builders/ci/fuchsia-ava-nelson/

Cq-Include-Trybots: luci.chrome.try:fuchsia-ava-nelson
Bug: 40935291
Change-Id: I12280f2e2e39d2ea21ac0aed43151341ef7cc18c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5731039
Commit-Queue: Zijie He <zijiehe@google.com>
Reviewed-by: David Song <wintermelons@google.com>
Cr-Commit-Position: refs/heads/main@{#1331887}

fuchsia_web/BUILD.gn

@@ -11,6 +11,7 @@ visibility = [ "//:gn_all" ]
 group("gn_all") {
   testonly = true
   deps = [
+    "av_testing:av_sync_tests",
     "runners:cast_runner",
     "shell",
     "webengine:web_engine",