[zlib] Run zlib_unittests on the buildbots

The zlib test suite has 96% coverage of the 'optimizations' folder
and is part of the development workflow (i.e. used to validate
correctness of zlib patches).

This change will ensure that the test suite will run as part of the
buildbots.

Bug: 1113596, 1124704
Change-Id: Icfcc88cfa5d1165549521d18915c743e45ceb782
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2390654


Reviewed-by: Adenilson Cavalcanti <cavalcantii@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Adenilson Cavalcanti <cavalcantii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804566}
GitOrigin-RevId: f7f838d1802b3aa91502b117cf4240070e65ffc4