test-lib-functions: use test-tool for [de]packetize()

test-lib-functions: use test-tool for [de]packetize()

The shell+perl "[de]packetize()" helper functions were added in
4414a150025 (t/lib-git-daemon: add network-protocol helpers,
2018-01-24), and around the same time we added the "pkt-line" helper
in 74e70029615 (test-pkt-line: introduce a packet-line test helper,
2018-03-14).

For some reason it seems we've mostly used the shell+perl version
instead of the helper since then. There were discussions around
88124ab2636 (test-lib-functions: make packetize() more efficient,
2020-03-27) and cacae4329fa (test-lib-functions: simplify packetize()
stdin code, 2020-03-29) to improve them and make them more efficient.

There was one good reason to do so, we needed an equivalent of
"test-tool pkt-line pack", but that command wasn't capable of handling
input with "\n" (a feature) or "\0" (just because it happens to be
printf-based under the hood).

Let's add a "pkt-line-raw" helper for that, and expose is at a
packetize_raw() to go with the existing packetize() on the shell
level, this gives us the smallest amount of change to the tests
themselves.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>