Fix autobuild for user names starting with c

Fix autobuild for user names starting with c

This is similar to the fix from commit 0b1ba00: Now commit 475a7616 introduced
tests that create user names in the form DOMAIN\USER and pass them through
shell functions. There "echo" is used to print he username, resulting in
/bin/dash on sn-devel to interpret e.g. \c which skips any further output. The
result are test exceptions like:

failure: samba4.blackbox.trust_ntlm.Test01 rpcclient lookupnames with LOCALADMEMBER\cs(ad_member:local) [
Exception: Exception: LOCALADMEMBER

]
time: 2018-02-26 23:00:46.688800Z

Fix this by replacing the echo with printf %s. This surfaced for
test_rpcclient_grep, but apply the same change to all functions in
common_test_fns.inc for consistency.

Signed-off-by: Christof Schmitt <cs@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Christof Schmitt <cs@samba.org>
Autobuild-Date(master): Fri Mar  2 10:49:09 CET 2018 on sn-devel-144