Changes

sccp: testenv: fix run with asan + latest binaries

When running against osmocom:nightly:asan, build sccp_demo_user with
--enable-sanitize. Otherwise this code is not running with asan and
doesn't even start (as the libraries we link against are built with
--enable-sanitize).

When running against osmocom:latest, check out the latest tag instead of
current master.

Fixes: OS#5899
Change-Id: I5307125560694feae9f0978ebd27607a77ed8675