Debugging tests

Use the tracing::[level]! macros for easier debugging.

The format for RUST_LOG is either

RUST_LOG=[level]

or

RUST_LOG=[target]=[level],[target]=[level],...

Where level can be one of error, warn, info, debug or trace. The target can be obtained from the list given by cargo test --package and by replacing dashes (-) with underscores (_) in the package names.

Ouisync uses tracing_subscriber to include information where individual output lines originated from. To enable this functionality, set the level to either info, debug or trace.