tests/test-gendoc-sv.t

histedt: use inheritance to override the constraints in 'base'

All actions but one actually have the same constraints when it comes to validate
the 'action.node' value. So we actually just add this code to a method that can
be overwritten in the one action where it matters.

The now unused 'contraints' related enum and class attribute will be cleaned up
in the next changeset.

#require docutils gettext

  $ $TESTDIR/check-gendoc sv
  checking for parse errors