test: reduce buffer size in buffer-creation test

This test is allocating much more memory than necessary to actually
reproduce the original problem. Lowering the amount of memory allocated
increases performance at least in some cases and makes this test less
likely to time out on SmartOS.

PR-URL: https://github.com/nodejs/node/pull/11177
Ref: https://github.com/nodejs/node/issues/10166



Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Julien Gilli <jgilli@nodejs.org>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>