[//gpu] Add tests of ClientSI::GetTextureTarget() with legacy MP

This CL adds tests of ClientSharedImage::GetTextureTarget() with legacy
multiplanar formats, checking that external sampling is correctly
detected.

Bug: 41494843
Change-Id: Ic137657edbb5864d451c8b2fa0884b4a0eeac172
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5412172


Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1281712}