selftests: netfilter: conntrack_sctp_collision.sh: move to lib.sh infra

selftests: netfilter: conntrack_sctp_collision.sh: move to lib.sh infra

While at it, address warnings generated by shellcheck and fix following
minor issues:

 - some distros place netem in 'extra' modules package, so add a skip check for netem-attach
   failure.
 - tc prints a warning for the 100mbit class:
   "Warning: sch_htb: quantum of class 10001 is big. Consider r2q change."
   Silence this by increasing the divisor.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240411233624.8129-7-fw@strlen.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>