tests/spi: add tostring check to interactive test

tests/test_spi.c

@@ -123,6 +123,7 @@ bool getc_yes(void) {
 }
 
 void test_interactive(void) {
+    char str[256];
     spi_t *spi;
     uint8_t buf[] = { 0x55, 0xaa, 0x0f, 0xf0 };
 
@@ -138,6 +139,12 @@ void test_interactive(void) {
     printf("Press enter to continue...\n");
     getc(stdin);
 
+    /* Check tostring */
+    passert(spi_tostring(spi, str, sizeof(str)) > 0);
+    printf("SPI description: %s\n", str);
+    printf("SPI description looks OK? y/n\n");
+    passert(getc_yes());
+
     /* Mode 0 transfer */
     printf("Press enter to start transfer...");
     getc(stdin);