tests: Extend SMB test_save_load_text case to check overwrite

tests: Extend SMB test_save_load_text case to check overwrite

Extend the test case to check overwriting a file as well. Currently this
has the behaviour of appending to the existing file, rather than
overwriting the file with new contents.

It's not clear from the API that this is the intended behaviour in this
case, so I've marked it as a failure.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13676

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>