s4/torture: add a creditting test skipping a SMB2 MID

s4/torture: add a creditting test skipping a SMB2 MID

This tests that skipping a SMB2 MID the client's usable MID window is

[unused mid, unused mid + 8192]

The test currently fails against Samba as we only grant up to 512
credits. It passes against Windows 2016 as that grants up to 8192
credits by default.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Sat Mar  4 01:54:07 CET 2017 on sn-devel-144