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/cp_files.c

@@ -0,0 +1,58 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <dirent.h>
+#include <errno.h>
+#include <string.h>
+
+int
+main(int argc, char *argv[])
+{
+	int tfd;
+	DIR *sdir;
+	struct dirent *dirent;
+
+	if (argc != 3) {
+		fprintf(stderr, "Usage: %s SRC DST\n", argv[0]);
+		exit(1);
+	}
+
+	sdir = opendir(argv[1]);
+	if (sdir == NULL) {
+		fprintf(stderr, "Failed to open %s: %s\n",
+		    argv[1], strerror(errno));
+		exit(2);
+	}
+
+	tfd = open(argv[2], O_DIRECTORY);
+	if (tfd < 0) {
+		fprintf(stderr, "Failed to open %s: %s\n",
+		    argv[2], strerror(errno));
+		closedir(sdir);
+		exit(3);
+	}
+
+	while ((dirent = readdir(sdir)) != NULL) {
+		if (dirent->d_name[0] == '.' &&
+		    (dirent->d_name[1] == '.' || dirent->d_name[1] == '\0'))
+			continue;
+
+		int fd = openat(tfd, dirent->d_name, O_CREAT|O_WRONLY, 0666);
+		if (fd < 0) {
+			fprintf(stderr, "Failed to create %s/%s: %s\n",
+			    argv[2], dirent->d_name, strerror(errno));
+			closedir(sdir);
+			close(tfd);
+			exit(4);
+		}
+		close(fd);
+	}
+
+	closedir(sdir);
+	close(tfd);
+
+	return (0);
+}