Fix key expiry check and add specs for some methods of the list model

Add specs for #export_key and #check_keys. When adding a spec for the lists key_check method, i noticed that the expiry check did not work, because of an incorrect variable name. Fixed with this MR.

Besides i adjusted the deletion of the files in the tmp GNUPGHOME. Now all folders are removed after the execution of the suite, because somehow not only one tmp-folder is created. That led to flaky tests, because sometimes a directory already existed. Now the deletion is done recursively.