[YUV AHB] Fix ExternalTextureTests quad drawing.

The tests were using 6 points to draw two triangles for a quad instead
of the traditional 3-point triangle that covers the whole viewport.
However some tests use Draw(3), which happened to work because they
checked a pixel 0, 0 out of a 4x4 texture. But when modifiying the tests
to be a 1x1 texture, the single triangle drawn would end up drawing
nothing.

Bug: 468988322
Change-Id: I01938e3850da489c08406b7aeb2fb94d5f48b270
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/298855
Reviewed-by: Brandon Jones <bajones@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>