media/gpu/vea_tests: Add test case to perform Flush() during encoding

Through media::VideoEncoder, VideoEncodeAccelerator::Flush() is
executed during encoding. This CL adds the test case to verify the
scenario.

Bug: b:386810873
Test: vea_tests on dedede and geralt
Change-Id: I420bcdeb524cfda102c0d0f3699c2355bcdd0c50
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6143239
Reviewed-by: Nathan Hebert <nhebert@chromium.org>
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1402487}