The vJoy C API is not thread-safe. This kind of error can occur: https://vjoy.freeforums.net/thread/28/call-registerclassex-failed-disabled-device.
By default Rust runs tests in parallel, so to avoid problems with thread safety we have two options:
1. Use cargo test -- --test-threads 1 to run every test serially.
2. Use serial_test crate and [serial] attribute.
The projects makes use of option 2.