test: move test-benchmark-buffer to sequential

`test-benchmark-buffer` has been observed to timeout on CI on SmartOS.
Move the test to `sequential` so it is not competing with other tests
for resources.

PR-URL: nodejs/node#15373
Fixes: nodejs/node#15372
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: James M Snell <jasnell@gmail.com>