pp_leaveeval: use EVAL_KEEPERR

pp_leaveeval: use EVAL_KEEPERR

In order shortly to allow pp_return to tail call pp_leaveval,
make pp_leaveval use the EVAL_KEEPERR flag bit of PL_in_eval
to decide whether to call CLEAR_ERRSV() rather than testing the
OPf_SPECIAL flag bit on the OP_LEAVEEVAL op (since we may no longer be
that op).

The two are equivalent (I checked this by running test test suite with
an assert that they were the same).

Note that pp_return already uses the EVAL_KEEPERR flag test, so this
commit makes the pp_return and pp_leavesub code blocks more similar.

I've also added some more tests, as I initially got the logic of this
wrong, and no core tests failed - only cpan/ ones.  In particular, no core
tests checked whether eval-string actually cleared $@ on success!