migrate staging tests

[jesseduffield]

• PR Description

Migrates the staging panel integration tests.

We need some better constructs for working with the staging panel, like being able to do assertions on the range of selected lines. We'll also need that for when we add the bulk actions feature. For now, I'm just sticking to the testing constructs we already have, with the focus being on moving away from the legacy pattern. But we'll need to come back and fix up this new tech debt.

• Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go run scripts/cheatsheet/main.go generate)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• Docs (specifically docs/Config.md) have been updated if necessary
• You've read through your own file changes for silly mistakes etc

[github-actions]

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