s3: smbtorture: Add SMB2-DFS-FILENAME-LEADING-BACKSLASH test.

s3: smbtorture: Add SMB2-DFS-FILENAME-LEADING-BACKSLASH test.

Shows that we fail to cope with MacOSX clients that send a
(or more than one) leading '\\' character for an SMB2 DFS pathname.

I missed this in earlier tests as Windows, Linux, and
libsmbclient clients do NOT send a leading backslash
for SMB2 DFS paths. Only MacOSX (sigh:-).

Passes against Windows. Adds a knownfail for smbd.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15277

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>