tests/subshell.sh: fix a test for systems without /dev/fd/*

ksh's built-in test, [ and [[ commands treat /dev/fd/* specially:
e.g. 'test /dev/fd/0' returns true even if it doesn't physically
exist, as on e.g. HP-UX. However, external commands need it to
exist physically.

src/cmd/ksh93/tests/subshell.sh:
- To decide whether to run a test with 'tee', use external 'test'
  command to check if /dev/stdout and /dev/fd/1 actually exist.

src/cmd/ksh93/tests/subshell.sh

@@ -625,7 +625,7 @@ $SHELL <<- \EOF
 EOF
 
 for f in /dev/stdout /dev/fd/1
-do	if	[[ -e $f ]]
+do	if	"${ whence -p test; }" -e "$f"
 	then	$SHELL -c "x=\$(command -p tee $f </dev/null 2>/dev/null)" || err_exit "$f in command substitution fails"
 	fi
 done