The test wasn't reserving a large enough buffer when initializing
the vertex data. Fix the test buffer limits.

Bug: angleproject:4430
Change-Id: I4b9a98e1af1cc8088afb871816061d7ee4dd6a2c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2079153Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>