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.: https://github.com/modernish/modernish/blob/3ddcbd13/lib/modernish/cap/BUG_KUNSETIFS.t
	https://github.com/modernish/modernish/blob/3ddcbd13/lib/modernish/tst/isset.t#L204-L222

NEWS

@@ -180,6 +180,8 @@ Any uppercase BUG_* names are modernish shell bug IDs.
   etc. to lose their effect after being unset in a subshell. For example:
     (unset PATH; PATH=/dev/null; ls); : wrongly ran 'ls'
     (unset LC_ALL; LC_ALL=badlocale); : failed to print a diagnostic
+  This also fixes BUG_KUNSETIFS: unsetting IFS in a subshell failed if IFS
+  was set to the empty value in the parent shell.
 
 - Fix crashes on some systems, including at least a crash in 'print -v' on
   macOS, by eliminating an invalid/undefined use of memccpy() on overlapping

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

src/cmd/ksh93/tests/variables.sh

@@ -268,6 +268,9 @@ fi
 for i in : % + / 3b '**' '***' '@@' '{' '[' '}' !!  '*a' '$foo'
 do      (eval : \${"$i"} 2> /dev/null) && err_exit "\${$i} not an syntax error"
 done
+
+# ___ begin: IFS tests ___
+
 unset IFS
 ( IFS='  ' ; read -r a b c <<-!
 	x  y z
@@ -418,6 +421,29 @@ do
 		;;
 	esac
 done
+
+# BUG_KUNSETIFS: Unsetting IFS fails to activate default default field splitting if two conditions are met:
+IFS=''		# condition 1: no split in main shell
+: ${foo-}	# at least one expansion is also needed to trigger this
+(		# condition 2: subshell (non-forked)
+	unset IFS
+	v="one two three"
+	set -- $v
+	let "$# == 3"	# without bug, should be 3
+) || err_exit 'IFS fails to be unset in subshell (BUG_KUNSETIFS)'
+
+# Test known BUG_KUNSETIFS workaround (assign to IFS before unset)
+IFS= v=
+: ${v:=a$'\n'bc$'\t'def\ gh}
+case $(unset IFS; set -- $v; print $#) in
+4 | 1)	# test if the workaround works whether we've got the bug or not
+	v=$(IFS=foobar; unset IFS; set -- $v; print $#)
+	[[ $v == 4 ]] || err_exit "BUG_KUNSETIFS workaround fails (expected 4, got $v)" ;;
+*)	err_exit 'BUG_KUNSETIFS detection failed'
+esac
+
+# ^^^ end: IFS tests ^^^
+# restore default split:
 unset IFS
 
 if	[[ $( (print ${12345:?}) 2>&1) != *12345* ]]