Adapt tests that rely on storage internals (login GUIDs, error messages) so they pass against both the JSON and the Rust backend. Also fixes an inconsistency between Rust's count and find methods and origin normalization. To run the password manager unit tests against the Rust backend: ``` ./mach xpcshell-test toolkit/components/passwordmgr/test/unit --setpref signon.storage.rust.enabled=true ``` Differential Revision: https://phabricator.services.mozilla.com/D306970