more integration test refactoring

[jesseduffield]

• PR Description

• adds switch-to-view methods which switch to a window and assert that the view on top of that window is the expected one
• allows chaining matchers e.g. Contains("blah").DoesNotContain("meme")
• Adds special IsSelected() matcher for use in the Lines and TopLines methods
• adds ViewAsserter struct for running assertions on views

• 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