Add regress test for fixed BUG_KUNSETIFS (re: 6f0e008c, 7b994b6a)

Modernish is no longer detecting BUG_KUNSETIFS, as I've just
discovered. Always nice when bugs spontaneously vanish...

A 'git reset HEAD~1'/recompile/retest loop reveals this bug was
fixed by 6f0e008c, as later modified by 7b994b6a.

So, let's make sure it stays fixed.

src/cmd/ksh93/tests/variables.sh:
- Add a couple of regression tests for BUG_KUNSETIFS presence,
  detection and known workaround, based on the same in modernish.
  Ref.: 3ddcbd13/lib/modernish/cap/BUG_KUNSETIFS.t
	3ddcbd13/lib/modernish/tst/isset.t (L204-L222)

TODO

@@ -71,12 +71,6 @@ https://github.com/modernish/modernish/tree/0.16/lib/modernish/cap/
   are erroneously interpreted as wildcards when quoted "$*" is used as the
   glob pattern.
 
-- BUG_KUNSETIFS: ksh93: Can't unset IFS under very specific circumstances.
-  unset -v IFS is a known POSIX shell idiom to activate default field
-  splitting. With this bug, the unset builtin silently fails to unset IFS
-  (i.e. fails to activate field splitting) if we're executing an eval or a
-  trap and a number of specific conditions are met.
-
 - BUG_LOOPRET2: If a 'return' command is given without a status argument
   within the set of conditional commands in a 'while' or 'until' loop (i.e.,
   between 'while'/'until' and 'do'), the exit status passed down from the