Add a setup script to be run from the Flutter customer test registry (#6690)

tool/flutter_customer_tests/setup.sh

diff --git a/tool/flutter_customer_tests/setup.sh b/tool/flutter_customer_tests/setup.sh
new file mode 100644
index 0000000..e9ca620
--- /dev/null
+++ b/tool/flutter_customer_tests/setup.sh
@@ -0,0 +1,9 @@
+#!/bin/bash -e
+# Script to generate mocks for Devtools from the flutter/tests registry
+# https://github.com/flutter/tests
+# This is executed as a pre-submit check for every PR in flutter/flutter
+
+cd tool
+flutter pub get
+dart bin/devtools_tool generate-code --upgrade
+cd ..