test_driver: check for leftover temp files after each test

test_driver: check for leftover temp files after each test

Reset the temp directory for every test to a local directory, then
after each test see if any new temp files were created and not
deleted: if they were then fail the test.  Rather than delete the
temp files we leave them there and avoid reporting files that were
seen before, so the user can investigate them.

Rewrite the temp_stdin tests to rely on this built-in behavior
rather than implementing the checks directly.

* tests/test_driver.pl: Create a $TEMPDIR variable pointing to a
temporary directory outside the test temp directory.
(toplevel) Before starting any tests create a temp directory and set
the POSIX and Windows temp directory environment variables to use it.
(compare_output) Check the contents of the temp directory.  If any
new files have appeared, fail the test.
* tests/scripts/features/temp_stdin: Remove check_tempfile() and
all users of it, plus setting of temp environment variables.