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 <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Gibson Fahnestock <[email protected]>

Author: aqrln