media/gpu/test: disable VideoDecoderTest.FlushMidStream test case

This test case is more trouble than it's worth, mostly because it's
complicated to guarantee that the flush happens at a good moment
codec-wise (and not e.g. in the middle on an H.264 GOP). This CL
disables it, effectively leaving it manual.

Bug: b:298028234
Change-Id: I3841a9edfcba801a390f34ac6c5c2ccf09fd0afc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4853416
Auto-Submit: Miguel Casas-Sanchez <mcasas@chromium.org>
Reviewed-by: Justin Green <greenjustin@google.com>
Commit-Queue: Justin Green <greenjustin@google.com>
Cr-Commit-Position: refs/heads/main@{#1194865}