tests/variables.sh: fix problems with tracing ('bin/shtests -x')

src/cmd/ksh93/tests/variables.sh:
- Unset -x in a command substitution that redirects stderr to
  stdout; this caused a spurious failure with tracing active.
- Execute the nameref tests in a subshell. They modified LINENO, so
  that all the line numbers after this test were traced and/or
  reported as 'foo'.
  . This exposed a bug in namerefs: this is yet another thing that
    is broken in non-forked/virtual subshells! That is for another
    commit. For now, fork the subshell and leave a TODO.

(cherry picked from commit 8b7f8f9b14523a66bdf337612daef2501c2bb5ba)

src/cmd/ksh93/tests/variables.sh

@@ -474,7 +474,7 @@ chmod +x $tmp/script
     (( .sh.subshell == 1 )) || err_exit ".sh.subshell not working in a subshell"
 )
 TIMEFORMAT='this is a test'
-[[ $({ { time :;} 2>&1;}) == "$TIMEFORMAT" ]] || err_exit 'TIMEFORMAT not working'
+[[ $(set +x; { { time :;} 2>&1;}) == "$TIMEFORMAT" ]] || err_exit 'TIMEFORMAT not working'
 : ${.sh.version}
 [[ $(alias integer) == *.sh.* ]] && err_exit '.sh. prefixed to alias name'
 : ${.sh.version}
@@ -541,6 +541,8 @@ esac
 	}
 } 2> /dev/null || err_exit "cannot add get discipline to .sh.foobar"
 [[ ${.sh.foobar} == world ]]  || err_exit 'get discipline for .sh.foobar not working'
+
+[[ -o xtrace ]] && opt_x=-x || opt_x=+x
 x='a|b'
 IFS='|'
 set -- $x
@@ -548,10 +550,11 @@ set -- $x
 exec 3>&2 2> /dev/null
 set -x
 ( IFS= ) 2> /dev/null
-set +x
+set "$opt_x"
 exec 2>&3-
 set -- $x
 [[ $2 == b ]] || err_exit '$2 should be b after subshell'
+
 : & pid=$!
 ( : & )
 [[ $pid == $! ]] || err_exit '$! value not preserved across subshells'
@@ -616,8 +619,12 @@ set -- {1..32768}
 (( $# == 32768 )) || err_exit "\$# failed -- expected 32768, got $#"
 set --
 
+(
+	# TODO: namerefs are broken in non-forked/virtual subshells.
+	# For now, fork it using ulimit; remove the ulimit to expose the test failures.
+	ulimit -t unlimited
+
 	unset r v x
-path=$PATH
 	x=foo
 	for v in EDITOR VISUAL OPTIND CDPATH FPATH PATH ENV LINENO RANDOM SECONDS _
 	do	nameref r=$v
@@ -642,7 +649,10 @@ do	nameref r=$v
 		{ g=$( r=C; r=$x; print -- $r ); } 2>/dev/null
 		[[ $g == 'C' ]] || err_exit "$v=C; $v=$x failed -- expected 'C', got '$g'"
 	done
-PATH=$path
+
+	exit $Errors
+)
+Errors=$?  # in case of failures, ensure error count survives subshell
 
 cd $tmp