selftests: firmware: Add request_firmware_into_buf tests

Add tests cases for checking request_firmware_into_buf api.
API was introduced into kernel with no testing present previously.

Signed-off-by: Scott Branden <scott.branden@broadcom.com>
Acked-by: Luis Chamberlain <mcgrof@kernel.org>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/20190822184005.901-3-scott.branden@broadcom.com


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