tests: Rework backup test_backup_invalid_args test-case

tests: Rework backup test_backup_invalid_args test-case

self.create_backup() uses self.run_cmd(), which is a wrapper around
self.check_output(). Rework the code to call the underlying
check_output() function directly instead.

The reason we're doing this is we want run_cmd() to catch exceptions and
fail the test (i.e. in the next patch). However, we can't do that because
this test case relies on receiving the exceptions.

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>