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/.gitignore

@@ -0,0 +1,34 @@
+/badsend
+/btree_test
+/chg_usr_exec
+/devname2devid
+/dir_rd_update
+/draid
+/file_check
+/file_trunc
+/file_write
+/get_diff
+/getversion
+/largest_file
+/libzfs_input_check
+/mkbusy
+/mkfile
+/mkfiles
+/mktree
+/mmap_exec
+/mmap_libaio
+/mmap_seek
+/mmapwrite
+/nvlist_to_lua
+/randfree_file
+/randwritecomp
+/read_dos_attributes
+/readmmap
+/rename_dir
+/rm_lnkcnt_zero_file
+/send_doall
+/stride_dd
+/threadsappend
+/user_ns_exec
+/write_dos_attributes
+/xattrtest