require the Test: field in commit messages

Bug: 29606287
Test: `repo upload` now requires Test: field in other repos
Change-Id: Ic465d211c2981f23f5d3d1b04153c70740942baa

GLOBAL-PREUPLOAD.cfg

@@ -1,3 +1,6 @@
 # Settings in here control defaults for `repo upload` checks for all projects
 # listed in the manifest.  Do not turn on things here without first consulting
 # the wider Android team, and make sure you send out a PSA before landing.
+
+[Builtin Hooks]
+commit_msg_test_field = true