tests/pty.sh: fix regress fail due to $TMPDIR

Test 137(C) was failing on some systems because $TMPDIR was set and
the local vi(1) honours it, so that the expected '/tmp/' string was
never output by vi. For compatibility with vi programs that honour
$TMPDIR and those that always use /tmp, we must export TMPDIR=/tmp.

src/cmd/ksh93/tests/pty.sh:

- Export TMPDIR=/tmp for test 137(C).
     Note that this exports TMPDIR to the environment for the
  duration of the 'tst' function run because the function was
  defined using the ksh 'function tst { ...; }' syntax.

src/cmd/ksh93/tests/pty.sh

@@ -19,6 +19,13 @@
 ########################################################################
 : : generated by mkptytests from "posix-sh-*-c.pty" : :
 
+# These are tests for the interactive shell, run in a pseudoterminal utility
+# called 'pty', which allows for scripting interactive sessions and which is
+# installed in arch/*/bin while building. To understand these tests, first
+# read the pty manual by running: arch/*/bin/pty --man
+
+# The # err_exit # comments are to enable shtests to count the tests.
+
 # the trickiest part of the tests is avoiding typeahead
 # in the pty dialogue
 
@@ -363,7 +370,7 @@ r history
 !
 
 # err_exit #
-tst $LINENO <<"!"
+TMPDIR=/tmp tst $LINENO <<"!"
 L POSIX sh 137(C)
 
 # If the User Portability Utilities Option is supported and shell