test-tool: Remove three casts

This patch does not change any functionality but removes the number
of casts in the test tool source code.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>

test-tool/iscsi-support.h

@@ -198,13 +198,13 @@ extern int sbc3_support;
 extern int maximum_transfer_length;
 
 struct scsi_device {
-	const char *error_str;
+	char *error_str;
 
 	struct iscsi_context *iscsi_ctx;
 	int iscsi_lun;
-	const char *iscsi_url;
+	char *iscsi_url;
 
-	const char *sgio_dev;
+	char *sgio_dev;
 	int sgio_fd;
 };
 extern struct scsi_device *sd;