fix, test, avoid writing references to the same file in convt.* regressions

This should facilitate parallel checks.

This fixes issue #2: https://bitbucket.org/hroptatyr/dateutils/issue/2/make-check-fails

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
Merge branch 'test/convt-tests'

* test/convt-tests:
  fix, test, avoid writing references to the same file in convt.* regressions