Adding a utest for small payloads

One of the optimizations (i.e. chunk_copy) will perform vector stores on
16 bytes chunks instead of the original 3 bytes scalar operations.

It is interesting to validate its safety while operating with small
payloads (i.e. data input smaller than a single load/store).

Even though it is a corner case (i.e. the payload would be smaller than
the wrapper used for the DEFLATE stream for GZIP), it is good to certify
that the optimization works as expected.

This will also add gtest as a dependency as the plan is to write some
tests to stress the optimizations we ship.

Bug: 1032721
Change-Id: Ifc6a81879e3dba6a9c4b7cfde80e7207258b934c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2128836


Commit-Queue: Adenilson Cavalcanti <cavalcantii@chromium.org>
Reviewed-by: Chris Blume <cblume@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#757639}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 272595ed5f469ee379e28dd5c40ef0230b6680a5