shtests: use central temporary directory; add --keep option

This gets rid of repetitive code in test scripts to create their
own temporary directories. Instead, shtests exports a $tmp to each
test script that is a subdirectory of its own temporary directory.
This has the advantage of having all test script temporary
directories in one hierarchy. Along with a new option to keep
temporary files, this makes it easy to inspect them if wanted.

This does make the test scripts less self-contained as they now
depend on a temporary directory being exported as $tmp. But they
already depended on $SHELL being the shell to test, so they already
were not quite self-contained.

src/cmd/ksh93/tests/shtests:
- Add -k/--keep option to keep temporary directory. Make the EXIT
  trap report its location instead of deleting it.
- For each test, create a subdirectory of $tmp (named after the
  test script plus the tested locale or 'shcomp') and export that
  subdirectory to the test script as its own $tmp.
- If -k is not given, delete each script's temporary files
  immediately after running it to minimise disk usage.

src/cmd/ksh93/tests/*.sh:
- Don't make own temp directory.
- Refuse to run if $tmp is not set.
- Miscellaneous tweaks.

src/cmd/ksh93/tests/enum.sh

@@ -27,6 +27,9 @@ alias err_exit='err_exit $LINENO'
 
 Command=${0##*/}
 integer Errors=0
+
+[[ -d $tmp && -w $tmp ]] || { err\_exit "$LINENO" '$tmp not set; run this from shtests. Aborting.'; exit 1; }
+
 enum Color_t=(red green blue orange yellow)
 enum -i Sex_t=(Male Female)
 for ((i=0; i < 1000; i++))