util_test: Use mkdtemp() instead of tempnam()
util_test: Use mkdtemp() instead of tempnam()

The latter function is considered insecure and deprecated on some platforms.
This will create the directory with 0700 implicitely, where it was 0755
beforehand. Since this is a more secure default and the test suite runs fine,
we don't bother to chmod the directory.

eworm-de/yubico-pam,eworm-de/yubico-pam,Yubico/yubico-pam,Yubico/yubico-pam,eworm-de/yubico-pam,Yubico/yubico-pam