Yes, it can do both of those things.

I've done a bunch of stuff with ChatGPT where I've pasted in code and asked it to make changes. The main limit there is the token limit - 8,000 I think for ChatGPT - so I'm very much looking forward to experimenting with the increased 32,000 limit when that becomes available.

I write tests for my code using Copilot a bunch - sometimes by pasting in the code I want to test and starting to type tests - it autocompletes them really effectively.

I wrote a bit about that here: https://til.simonwillison.net/gpt3/writing-test-with-copilot