Always copy return values when exiting scope

Always copy return values when exiting scope

v5.14.0-642-g3ed94dc fixed certain instances where returning from a sub
incorrectly returned the actual value rather than a copy, e.g.

    sub f { delete $foo{bar} }

This was because if the value(s) being returned had SvTEMP set, copying
was skipped. That commit added an extra condition to the skip test,
SvREFCNT(sv) == 1.

However, this applies equally well to other scope exits, for example

    do { ...; delete $foo{bar} }

So this commits adds the RC==1 test to S_leave_common() too, which handles
all the non-sub scope exits. As well as adding a test to do.t, it adds an
additional test to sub.t, since the original tests, although they
*detected* a non-copied return, didn't actually demonstrate a case where
it was actually harmful.

Note that S_leave_common() also sometimes skips on PADTMPs as well as
TEMPs, so this commit as a side-effect also makes it copy PADTMPs unless
their RC ==1. But since their RC should in fact almost always be 1 anyway,
in practice it makes no difference.