usb: Update the test to cover reading and writing

Add test coverage for blk_write() as well.

The blk_erase() is not tested for now as the USB stor interface does not
support erase.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/dm/usb.c b/test/dm/usb.c
index 445b21a..7671ef1 100644
--- a/test/dm/usb.c
+++ b/test/dm/usb.c
@@ -61,7 +61,24 @@
 	ut_asserteq(512, dev_desc->blksz);
 	memset(cmp, '\0', sizeof(cmp));
 	ut_asserteq(2, blk_read(blk, 0, 2, cmp));
-	ut_assertok(strcmp(cmp, "this is a test"));
+	ut_asserteq_str("this is a test", cmp);
+
+	strcpy(cmp, "another test");
+	ut_asserteq(1, blk_write(blk, 1, 1, cmp));
+
+	memset(cmp, '\0', sizeof(cmp));
+	ut_asserteq(2, blk_read(blk, 0, 2, cmp));
+	ut_asserteq_str("this is a test", cmp);
+	ut_asserteq_str("another test", cmp + 512);
+
+	memset(cmp, '\0', sizeof(cmp));
+	ut_asserteq(1, blk_write(blk, 1, 1, cmp));
+
+	memset(cmp, '\0', sizeof(cmp));
+	ut_asserteq(2, blk_read(blk, 0, 2, cmp));
+	ut_asserteq_str("this is a test", cmp);
+	ut_asserteq_str("", cmp + 512);
+
 	ut_assertok(usb_stop());
 
 	return 0;