lib/libc/tests/rpc: Correctly set timeout

The rpc_control() API does not accept the CLCR_SET_RPCB_TIMEOUT command,
it only accepts RPC_SVC_CONNMAXREC_GET/RPC_SVC_CONNMAXREC_SET, so it was
not doing anything.
Instead of incorrectly calling this API, use clnt_create_timed() instead.

I noticed this because the test was timing out after 120s in the CheriBSD CI.

Reviewed By: ngie
Differential Revision: https://reviews.freebsd.org/D28478