Lower precision in Vulkan caused off-by-one issues with this test.

BUG=angleproject:4438

Change-Id: I17111a06133ef403be1f0c99321f1dbf8ab6083c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2078791Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>