[//gpu] Simplify ClientSI unittests

We now have a utility function to emulate using a client-provided GMB,
so we can use that function in the external sampling tests rather than
having them configure ClientSI to allow external sampling without
client-provided native buffers.

Bug: 41494843
Change-Id: I6d28af01716698015928ddbeb71ca3423227e7be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5418874


Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1281882}