selftests: mptcp: print trailing bytes with od

This is better than printing random bytes in the terminal.

Note that Jakub suggested 'hexdump', but Mat found out this tool is not
often installed by default. 'od' can do a similar job, and it is in the
POSIX specs and available in coreutils, so it should be on more systems.

While at it, display a few more bytes, just to fill in the two lines.
And no need to display the 3rd only line showing the next number of
bytes: 0000040.

Suggested-by: Jakub Kicinski <[e-mail]>
Suggested-by: Mat Martineau <[e-mail]>
Reviewed-by: Mat Martineau <[e-mail]>
Reviewed-by: Geliang Tang <[e-mail]>
Signed-off-by: Matthieu Baerts (NGI0) <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/net/mptcp/mptcp_lib.sh

···
 mptcp_lib_print_file_err() {
 	ls -l "${1}" 1>&2
 	echo "Trailing bytes are: "
-	tail -c 27 "${1}"
+	tail -c 32 "${1}" | od -x | head -n2
 }
 
 # $1: input file ; $2: output file ; $3: what kind of file