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: #15373
Fixes: #15372
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Ruben Bridgewater <[email protected]>
Reviewed-By: Evan Lucas <[email protected]>
Reviewed-By: Refael Ackermann <[email protected]>
Reviewed-By: Yuta Hiroto <[email protected]>
Reviewed-By: James M Snell <[email protected]>

Author: Trott