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: #16775
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Gireesh Punathil <[email protected]>
Reviewed-By: Khaidi Chu <[email protected]>
Reviewed-By: James M Snell <[email protected]>

Author: Trott