dist-modules: remove perl 5.8, add 5.18

This was failing for threads.pm, threads-shared, Thread-Semaphore
under 5.8.9 threaded.

I wasn't able to make these fail with my local copy of 5.8.9 threaded.

These are failing with messages like:

  Unbalanced string table refcount: (0) for "TRAP" during global destruction.
  Unbalanced string table refcount: (0) for "ERRNO" during global destruction.
  Unbalanced string table refcount: (0) for "setuid" during global destruction.
  Unbalanced string table refcount: (0) for "POSIX.pm" during global destruction.
  Unbalanced string table refcount: (0) for "SKIP" during global destruction.
  Unbalanced string table refcount: (0) for "load" during global destruction.
  Unbalanced string table refcount: (0) for "register::" during global destruction.
  Unbalanced string table refcount: (0) for "_filters" during global destruction.
  Unbalanced string table refcount: (0) for "gen_uid" during global destruction.
  Unbalanced string table refcount: (0) for "==" during global destruction.
  Unbalanced string table refcount: (0) for "FINALIZED" during global destruction.
  Unbalanced string table refcount: (0) for "event_tap" during global destruction.
  Unbalanced string table refcount: (0) for "confess" during global destruction.
  Unbalanced string table refcount: (0) for "CAN_SIGSYS" during global destruction.
  Unbalanced string table refcount: (0) for "NUM56" during global destruction.
  Unbalanced string table refcount: (0) for "tv_interval" during global destruction.

Author: tonycoz

.github/workflows/testsuite.yml

@@ -819,8 +819,8 @@ jobs:
         perl-version:
           - '5.38'
           - '5.24'
+          - '5.18'
           - '5.10'
-          - '5.8'
         threads:
           - enable: true
             label: -Dusethreads