Fix unintialized data in VertexAttributeTest.
Fix unintialized data in VertexAttributeTest.

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/+/2505551
Reviewed-by: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>
Commit-Queue: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle