Fix for failing unit-tests (which fire during cpan installation, making it difficult to install the module)

Resolved the problem of:

Failed test 'read a pwsafe3 database and change a record (Insecure dependency in unlink while running with -T switch at /home/marcink/DEV_hg/perl_patches/Crypt--PWSafe3/blib/lib/Crypt/PWSafe3.pm line 330.
  )'   at t/run.t line 66.

while fixing, introduced slightly better way to create temporary file (previous method
had minimal chance of clashing the name with existing file which would be removed, now we create unique temporary directory).

Note: if temporary name need not be generated strictly this way, it may be better to just use tempfile...