Rename Stencil_Test to stencil_operations, a more fitting name for a
sample.

BUG=angle:521

Change-Id: I5412ecef12d80c3dbf54539bc7509678a35c44d5
Reviewed-on: https://chromium-review.googlesource.com/188728Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>