Add E2E/Integration tests

[Lms24]

Now that we have quite a few wizard flows with many of them supporting advanced features (Script/AST modification, file merging, etc). We should add some e2e/integration-like tests that cover most flows.

Right now I'm thinking about a test setup that supports something similar to this:

• easily add scenarios/fixtures for whatever wizard flow
• each scenario consists of:
  • input files (e.g. bundler configs/minimal starter apps for a certain framework)
  • one or multiple tests that simulate interacting with a CLI
  • alternatively, we mock clack (or our prompt functions) and simulate user input
  • snapshots for output files or good assertions on output files
• Sentry API calls are mocked for simplicity

[github-actions[bot]]

This issue has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you label it Status: Backlog or Status: In Progress, I will leave it alone ... forever!

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