test: move timing-dependent tests to sequential

Move test-http-server-keep-alive-timeout-slow-server and
test-http-server-keep-alive-timeout-slow-client-headers from parallel to
sequential to resolve test flakiness on freebsd10-64.

Fixes: #14033
Refs: #9317
PR-URL: #14377
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>

Author: aqrln