Repeated keystore creations leak file descriptors

With sequoia-keystore 0.7.0, on macOS, the following

    #[test]
    fn repeated_keystore() {
        let keystore_dir = tempfile::tempdir().unwrap();
        let context = sequoia_keystore::Context::configure()
            .home(&keystore_dir.path())
            .build()
            .unwrap();
        for _ in 0..1000 {
            let keystore = match sequoia_keystore::Keystore::connect(&context) {
                Ok(m) => m,
                Err(e) => {
                    eprintln!("FAILED: PID {}: {}", std::process::id(), e);
                    std::thread::sleep(std::time::Duration::from_secs(1000)); // To allow listing open files
                    unreachable!()
                }
            };
            drop(keystore);
        }
    }

fails, typically with “Too many open files”. lsof shows many kqueue file descriptors, and some TCP sockets listening on localhost ports.

(Filing this early, in case the cause were obvious to someone. I’ll continue investigating.)

Cc: @dueno