tests/basic.sh: fix intermittent spurious regress fail

~- and ~+ are ksh93-specific tilde expansions that expand to
$OLDPWD and $PWD, respectively. On some systems, $OLDPWD is not set
on entry to the test script, because it is not exported to the
environment. This made it unset before any 'cd' was executed,
which (correctly) disabled ~- expansion.

src/cmd/ksh93/tests/basic.sh:
- Before testing 'cd ~-', make sure $OLDPWD is set by cd'ing to
  /dev first (a directory guaranteed by POSIX).

src/cmd/ksh93/tests/basic.sh

@@ -121,7 +121,10 @@ then	set -- TT*
 	then	err_exit 'TT* not expanding when file TT* exists'
 	fi
 fi
+
+cd /dev
 cd ~- || err_exit "cd back failed"
+
 cat > $tmp/script <<- !
 	#! $SHELL
 	print -r -- \$0