test: move test-http-keepalive-maxsockets to sequential

test-http-keepalive-maxsockets.js will fail if sufficient copies are run
at once. Move to sequential.

PR-URL: #16777
Reviewed-By: Anatoli Papirovski <[email protected]>
Reviewed-By: Refael Ackermann <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Gireesh Punathil <[email protected]>
Reviewed-By: Yuta Hiroto <[email protected]>
Reviewed-By: James M Snell <[email protected]>

Author: Trott