libip6t_srh.t: switch to lowercase, add /128 suffix, require success

libip6t_srh.t: switch to lowercase, add /128 suffix, require success

This looks like an oversight which is easy to fix.

Furthermore:
  git grep ';;OK'
does not find any other matches, so this is the last unverified test case.

Test:
  [root@f32vm IPT]# uname -r
  5.6.10-300.fc32.x86_64

  [root@f32vm IPT]# md5sum extensions/libip6t_srh.t
  b98864bdd6c39a0dd96022c47e652edb  extensions/libip6t_srh.t

  [root@f32vm IPT]# ./iptables-test.py extensions/libip6t_srh.t
  extensions/libip6t_srh.t: OK
  1 test files, 27 unit tests, 27 passed

Signed-off-by: Maciej Żenczykowski <maze@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>