s4: torture: Add an async SMB2_OP_FLUSH + SMB2_OP_FLUSH test to smb2.compound_async.

s4: torture: Add an async SMB2_OP_FLUSH + SMB2_OP_FLUSH test to smb2.compound_async.

Shows we fail sending an SMB2_OP_FLUSH + SMB2_OP_FLUSH
compound if we immediately close the file afterward.

Internally the flushes go async and we free the req, then
we process the close. When the flushes complete they try to access
already freed data.

Extra test which will allow me to test when the final
component (flush) of the compound goes async and returns
NT_STATUS_PENDING.

Add knownfail.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15172

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>