rpc: fix test broken by CL/8140
rpc: fix test broken by CL/8140

The test still assumed the existance of config.CertPool which was set to
nil in the linked CL. The change initializes it to an empty cert pool
instead.

https://upspin-review.googlesource.com/c/8140

Change-Id: If201323742f1c0536922333e50ed36e15441bce5
Reviewed-on: https://upspin-review.googlesource.com/8302
Reviewed-by: Eric Grosse <3706282e8b8557d7b3733fe2b8fbbcffca452208@gmail.com>
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

upspin/upspin,upspin/upspin