pad.c:padlist_dup: Remove refcnt check

This was added by 6de654a5, and the assert that makes sure the
reference count is exactly 1 was added in the same commit.  After
several years, I think we can be sure now that the reference count
is indeed always 1.  We don’t need to ‘play it safe’ for non-debug-
ging builds.
Fix UTF8 lex sub names

UTF8 lexical sub names were getting mangled, with extra junk on the end,
due to a precedence problem.
Make pad names always UTF8

Prior to 5.16, pad names never used the UTF8 flag, and all non-ASCII
pad names were in UTF8.  Because the latter was consistently true,
everything just worked anyway.

In 5.16, UTF8 handling was done ‘properly’, so that non-ASCII UTF8
strings were always accompanied by the UTF8 flag.

Now, it is still the case that the only non-ASCII names to make their
way into pad name code are in UTF8.  Since ASCII is a subset of UTF8,
we effectively *always* have UTF8 pad names.  So the flag handling is
actually redundant.

If we just assume that all pad names are UTF8 (which is true), then
we don’t need to bother with the flag checking.  There is actually
no reason why we should have two different encodings for storing
pad names.

So this commit enforces what has always been the case and removes the
extra code for converting between Latin-1 and UTF8.  Nothing on CPAN
is using the UTF8 flag with pads, so nothing should break.  In fact,
we never documented padadd_UTF8_NAME.
pad.c: Use UTF8f for ‘will not stay shared’

This is more efficient than creating a temporary SV.
diag.t: Allow multiline diag_listed_as
‘Subroutine (not var) "&x" will not stay shared’

Another ‘variable’ warning about lexical subs that I missed.
fetch_pad_names.t: Emit all test names

They were defined in the structure passed to the general_tests func-
tion, but not all of them were used.