Add regression tests

[rbren]

End-user friendly description of the problem this fixes or functionality that this introduces

• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below
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Give a summary of what the PR does, explaining any non-trivial design decisions

This is a POC for some regression tests we could add, to ensure OH stays good at certain tasks. The broad idea is to bridge the gap between the benchmarks we have, and problems we uncover just by playing with it.

Here's a list of things I'd like to have regression cases for, probably multiple for each:

• fix merge conflicts
• fix lint
• fix failing unit test
• add test coverage to function
• ability to install a project
• ability to run tests
• interactions with interactive CLIs
• user request updates
  • "check out a branch foobar"
  • "actually call it foobarbaz"
• creating .openhands instructions
• installing docker
• installing java

Hypothetically, some of these bullets could turn into a benchmark if we can find enough cases of that issue in the wild.

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Link of any specific issues this addresses

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To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:e4d870f-nikolaik   --name openhands-app-e4d870f   docker.all-hands.dev/all-hands-ai/openhands:e4d870f