Roll customer_testing from 8be7209 to c9cbc60 (4 commits) (flutter#167754)

This pulls in the 4 commits below:

flutter/tests@8be7209...c9cbc60

2025-04-24 greg@zulip.com Bump Zulip pin to update sliver z-order tests
(flutter/tests#456) 2025-04-17
43759233+kenzieschmoll@users.noreply.github.com Revert "Disable
flutter_devtools.test" (flutter/tests#455) 2025-04-15 aam@google.com
Update zulip hash to pick up version that can be analyzed cleanly.
(flutter/tests#453) 2025-04-15 bkonyi@google.com Disable
flutter_devtools.test (flutter/tests#452)

(using the command from 5ce5251 / flutter#162763 to make the log).

The two changes from 2025-04-15 had been here since db68c95 (flutter#16721),
but then it looks like they were accidentally reverted earlier today by
417f47f (flutter#165596).

The change from 04-17 doesn't appear to have made it here to
flutter/flutter yet.

The latest change just went into flutter/tests today and is why I'm
making this PR.

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Author: gnprice

dev/customer_testing/tests.version

@@ -1 +1 @@
-8be72094d9b33dae8ff8a7085a3d922e7b3cad47
+c9cbc600e3974fb5f2fe505def53f1dc11cefda7