chromeos-mash: disable UserAddingScreenTest.AddingSeveralUsers

This test is flaky on the msan bot, and I suspect it could flake on the
non-msan bot as well.

BUG=891383
TEST=test only change

Change-Id: Ib8bdc7248faab8058fb8c37a02529214cb580f91
Reviewed-on: https://chromium-review.googlesource.com/c/1256987Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595925}

testing/buildbot/filters/chromeos.single_process_mash.browser_tests.filter

@@ -232,6 +232,10 @@
 # Flaky tests. crbug.com/880584
 -UnifiedAutoplaySettingBrowserTest.*
 
+# This seems to timeout only on the msan bot, but I suspect it could equally
+# occur on the non-msan bots. https://crbug.com/891383
+-UserAddingScreenTest.AddingSeveralUsers
+
 # Window Open API tests.  https://crbug.com/815379
 -WindowOpenApiTest.OpenLockedFullscreenWindow
 -WindowOpenApiTest.RemoveLockedFullscreenFromWindow