Adds target for ash_mus_unittests filter file

testing/buildbot/filters/BUILD.gn

Index: testing/buildbot/filters/BUILD.gn
diff --git a/testing/buildbot/filters/BUILD.gn b/testing/buildbot/filters/BUILD.gn
index 7c54b682ae63a3157e7e0589c56d29ff24d416f8..bb2115cadb2e0074133ef5b773f206e5a89db90a 100644
--- a/testing/buildbot/filters/BUILD.gn
+++ b/testing/buildbot/filters/BUILD.gn
@@ -44,3 +44,11 @@ source_set("interactive_ui_tests_filters") {
     "//testing/buildbot/filters/site-per-process.interactive_ui_tests.filter",
   ]
 }
+
+source_set("ash_unittests_filters") {
+  testonly = true
+
+  data = [
+    "//testing/buildbot/filters/ash_mus_unittests.filter",
+  ]
+}