test_firmware: add support for firmware_request_platform

Add support for testing firmware_request_platform through a new
trigger_request_platform trigger.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Luis Chamberlain <mcgrof@kernel.org>
Link: https://lore.kernel.org/r/20200115163554.101315-6-hdegoede@redhat.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>