selftests/net: packetdrill: import tcp/slow_start

selftests/net: packetdrill: import tcp/slow_start

Same import process as previous tests.

Also add CONFIG_NET_SCH_FQ to config, as one test uses that.

Same test process as previous tests. Both with and without debug mode.
Recording the steps once:

make mrproper
vng --build \
        --config tools/testing/selftests/net/packetdrill/config \
        --config kernel/configs/debug.config
vng -v --run . --user root --cpus 4 -- \
	make -C tools/testing/selftests TARGETS=net/packetdrill run_tests

Link: https://github.com/linux-netdev/nipa/wiki/How-to-run-netdev-selftests-CI-style#how-to-build
Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20240912005317.1253001-4-willemdebruijn.kernel@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>