Copy RE capture buf on overload as well as TEMP

Copy RE capture buf on overload as well as TEMP

Partial fix for [perl #77084]. Sometimes pp_match makes a copy of the
original SV's string for the later use of $1 et al; in particular if the
SV is TEMP (so will soon go away).

Make it do the same if the SV is overloaded, as the string return is most
certainly temporary!

(Also tweak the tests to make them more likely to fail on badness by
creating new stings that will likely reallocate freed buffer).