selftests: firmware: Add firmware_request_platform tests

Add tests cases for checking the new firmware_request_platform api.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Luis Chamberlain <mcgrof@kernel.org>
Link: https://lore.kernel.org/r/20200115163554.101315-7-hdegoede@redhat.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>