tests: move C test helpers into test cmd

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13259

tests/zfs-tests/cmd/zfs_diff-socket.c

@@ -0,0 +1,57 @@
+/*
+ * This file and its contents are supplied under the terms of the
+ * Common Development and Distribution License ("CDDL"), version 1.0.
+ * You may only use this file in accordance with the terms of version
+ * 1.0 of the CDDL.
+ *
+ * A full copy of the text of the CDDL should have accompanied this
+ * source.  A copy of the CDDL is also available via the Internet at
+ * http://www.illumos.org/license/CDDL.
+ */
+
+/*
+ * Copyright 2017, loli10K <ezomori.nozomu@gmail.com>. All rights reserved.
+ */
+
+#include <fcntl.h>
+#include <sys/un.h>
+#include <sys/socket.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
+#include <errno.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+int
+main(int argc, char *argv[])
+{
+	struct sockaddr_un sock;
+	int fd;
+	char *path;
+	size_t size;
+	if (argc != 2) {
+		fprintf(stderr, "usage: %s /path/to/socket\n", argv[0]);
+		exit(1);
+	}
+	path = argv[1];
+	size =  sizeof (sock.sun_path);
+	strncpy(sock.sun_path, (char *)path, size - 1);
+	sock.sun_path[size - 1] = '\0';
+
+	sock.sun_family = AF_UNIX;
+	if ((fd = socket(AF_UNIX, SOCK_DGRAM, 0)) == -1) {
+		perror("socket");
+		return (1);
+	}
+	if (bind(fd, (struct sockaddr *)&sock, sizeof (struct sockaddr_un))) {
+		perror("bind");
+		return (1);
+	}
+	if (close(fd)) {
+		perror("close");
+		return (1);
+	}
+	return (0);
+}