test: improve code coverage of diagnostics_channel
test: improve code coverage of diagnostics_channel
Current coverage report: https://app.codecov.io/gh/nodejs/node/blob/main/lib%2Fdiagnostics_channel.js
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
test: improve code coverage of diagnostics_channel
Current coverage report: https://app.codecov.io/gh/nodejs/node/blob/main/lib%2Fdiagnostics_channel.js