tests/unit/test-smp-parse: Add 'smp-generic-invalid' machine type

Avoid modifying the MachineClass internals by adding the
'smp-generic-invalid' machine, which inherits from TYPE_MACHINE.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Yanan Wang <wangyanan55@huawei.com>
Message-Id: <20211216132015.815493-5-philmd@redhat.com>