testrun.sh: Add support for --tool=rpctrace

rpctrace(1) is a Hurd RPC tracer tool, which is used similar to how
strace(1) is used on GNU/Linux.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
Message-Id: <20210907133325.255690-6-bugaevc@gmail.com>
Acked-by: Florian Weimer <fweimer@redhat.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>