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: https://github.com/nodejs/node/pull/16777


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: James M Snell <jasnell@gmail.com>