Test: Output appending improvements

[isidorn]

• any os - @kieferrm
• any os - @sandy081

As per #22804 we have made improvements in how we are appending content to the output channels. Verify:

• All output channels behave as before and that the output select dropdown nicely reflects the currently opened output
• You can create / delete new output channels from an extension and send output to that channel
• When there is too much output we show the last recieved output and stop showing the oldest
• You can spam the output from an extension and vscode will not crash (though it might be unresponsive for a couple of seconds due to JSON parseing and sending)

[kieferrm]

@isidorn This is much improved over v1.10 particularly for single channel output! I'm surprised that I still get around 73'000 smaller output lines (message #44444) in a channel before it is being truncated.