Allow slop on a few locale tests

Four recently introduced tests in locale.t fail for two locales of all
the ones that get tested in our smoke farm.  I investigated the failures
and it looks to me like the problem in each case is that the locale
definition is defective.

The tests were added because of finding and fixing a bug in Perl, so I
don't want to remove them.  Instead these 4 tests will be marked as TODO
if at least 95% of locales pass on any given machine.

This works for our current smokers.
perlapi: Clarify av_pop

This notes that the caller now has control of a reference count of the
returned SV.

Wording mostly suggested by Paul Evans