SPI: add test endpoint for arbitrary lengths (#1949)

Co-authored-by: Comma Device <device@comma.ai>

board/drivers/spi.h

@@ -182,6 +182,10 @@ void spi_rx_done(void) {
         } else {
           print("SPI: did expect data for can_write\n");
         }
+      } else if (spi_endpoint == 0xABU) {
+        // test endpoint, send max response length
+        response_len = spi_data_len_miso;
+        response_ack = true;
       } else {
         print("SPI: unexpected endpoint"); puth(spi_endpoint); print("\n");
       }