test: move timing-sensitive test to sequential

test-https-server-keep-alive-timeout relies on server timeouts and
whatnot that will be inherently unreliable on a busy host. The test
fails when run with a high `-j` value and higher `--repeat` value passed
to `tools/test.py`. Move the test to `sequential`.

PR-URL: https://github.com/nodejs/node/pull/16775


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Khaidi Chu <i@2333.moe>
Reviewed-By: James M Snell <jasnell@gmail.com>