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/.gitignore

@@ -32,3 +32,14 @@
 /user_ns_exec
 /write_dos_attributes
 /xattrtest
+/zed_fd_spill-zedlet
+/suid_write_to_file
+/cp_files
+/ctime
+/truncate_test
+/ereports
+/zfs_diff-socket
+/dosmode_readonly_write
+/edonr_test
+/skein_test
+/sha2_test