Restore buffered process.PipeOutput and add tests

Created by: mrnugget

This restores the buffered PipeOutput function that I accidentally removed in #31081 because I didn't notice it is used in src-cli.

This adds it back and also adds a tests for both methods so we're sure that they actually do what they say they do.

Test plan

• Added and ran two unit tests, both with -count=20 to avoid race conditions/flakes
• Tests behaviour in src-cli by having github-proxy print single characters and running go run . run github-proxy in ./dev/sg