tests: cmd: don't recurse

This confers an >10x speedup on t/z-t/cmd builds (12s -> 1.1s),
gets rid of 23 redundant identical automake specs and gitignores,
and groups the binaries with their common headers

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/getversion.c

@@ -0,0 +1,48 @@
+/*
+ * 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 2021 iXsystems, Inc.
+ */
+
+/*
+ * FreeBSD and macOS expose file generation number through stat(2) and stat(1).
+ * Linux exposes it instead through an ioctl.
+ */
+
+#include <sys/ioctl.h>
+#include <sys/fcntl.h>
+#include <linux/fs.h>
+#include <err.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+int
+main(int argc, const char * const argv[])
+{
+	if (argc != 2)
+		errx(EXIT_FAILURE, "usage: %s filename", argv[0]);
+
+	int fd = open(argv[1], O_RDONLY);
+	if (fd == -1)
+		err(EXIT_FAILURE, "failed to open %s", argv[1]);
+
+	int gen = 0;
+	if (ioctl(fd, FS_IOC_GETVERSION, &gen) == -1)
+		err(EXIT_FAILURE, "FS_IOC_GETVERSION failed");
+
+	(void) close(fd);
+
+	(void) printf("%d\n", gen);
+
+	return (EXIT_SUCCESS);
+}