The test was initializing an extra element at the end of a vertex
buffer and never initializing it. I fixed this for one test but
the bug was actually replicated into several. This fixes the bug
by inializing the last vertex to the zero vertex value.

Bug: angleproject:3951
Bug: angleproject:4163
Bug: angleproject:4258
Bug: angleproject:4269
Bug: angleproject:5258
Change-Id: I226406475340d756bfb7624d163abeca7b807f20
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2505551Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>