wantarray.t: Remove unused var

It stopped being used in dab34d0f01.
wantarray.t: Test logops at sub exit
wantarray.t: $got before $expected

When the tests fail, the diagnostic output is very confusing
otherwise.
Use GIMME_V in preference to GIMME

GIMME_V is a simpler macro that results in smaller machine code.

GIMME does not distinguish between scalar and void context.  The two
instances of GIMME == G_SCALAR that I changed (which used to match
void context too, but no longer do) are in code paths unreachable in
void context, so we don’t need to check for it.
pp_ctl.c: Only do GIMME_V once in pp_caller

This shrinks the machine code.

Before and after:

$ ls -l pp_ctl.o
-rw-r--r--  1 sprout  staff  96156 Dec 19 16:05 pp_ctl.o
$ ls -l pp_ctl.o
-rw-r--r--  1 sprout  staff  96044 Dec 19 16:09 pp_ctl.o
pp_sys.c:pp_readdir: Call GIMME_V just once
universal.c:re_regexp_pattern: Mention GIMME_V once

While it was only being called once, it occurred in two code paths.
Pulling it out of the main if-block reduces the size of universal.o.
Before and after:

$ ls -l universal.o
-rw-r--r--  1 sprout  staff  33700 Dec 19 16:05 universal.o
$ ls -l universal.o
-rw-r--r--  1 sprout  staff  33692 Dec 19 16:13 universal.o
[perl #123458] list cx re::regexp_pattern($nonre)

It was returning (undef) in list context, though it was documented to
return the empty list.