Bump customer testing version for flutter/devtools update (#193511)

Bumps the customer testing pinned commit from `9853d38` to
[`51bbf1a`](https://github.com/flutter/tests/commit/51bbf1a5e9d4e0c53879fb52d3caacdd22d20304)
to pick up:
- https://github.com/flutter/tests/pull/496 (updates DevTools to
https://github.com/flutter/devtools/pull/10016)

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diff --git a/dev/customer_testing/tests.version b/dev/customer_testing/tests.version
index b09829b..e702028 100644
--- a/dev/customer_testing/tests.version
+++ b/dev/customer_testing/tests.version
@@ -1 +1 @@
-9853d386f609dc41aa22648131756f4eb69fee70
+51bbf1a5e9d4e0c53879fb52d3caacdd22d20304