Bump customer tests for zulip fix 2 (flutter#173003)

For flutter#165832, picks up the test
change from zulip/zulip-flutter#1761 that was
bumped in the tests repo in flutter/tests#466.

Author: justinmc

dev/customer_testing/tests.version

@@ -1 +1 @@
-5537bf576df9bbf27f7abc43a20b3f173e80f067
+20af51f6c62f7134c0c387d0ec2e13db33b1b9e4