test: find way to test `Layer`

[CrabNejonas]

Currently testing the Layer is difficult bc the data is generated through tracing macros, tracing-mock seems promising here, but can it integrate with a custom layer?

[CrabNejonas]

#41 includes tests for Layer, but they are broken right now bc apparently tracing (inc combination with tokio?) leaks events cross-thread even if we explicitly set the subscriber to be current thread.