CVE-2023-42669 s3-rpc_server: Disable rpcecho for consistency with the AD DC

CVE-2023-42669 s3-rpc_server: Disable rpcecho for consistency with the AD DC

The rpcecho server in source3 does have samba the sleep() feature that
the s4 version has, but the task architecture is different, so there
is not the same impact.  Hoever equally this is not something that
should be enabled on production builds of Samba, so restrict to
selftest builds.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15474

Signed-off-by: Andrew Bartlett <abartlet@samba.org>