Fix #51: "unit tests leave a bunch of /tmp/logcabinXXXXXX dirs"

This was caused by a few instances of:
  tmpdir.close();
  FilesystemUtil::remove(tmpdir.path);
Since tmpdir.close() clears tmpdir.path, this wouldn't remove anything.
The easy fix is to close tmpdir after removing the directory, or in all
the cases involved, just let tmpdir's destructor close it.