fix(cli): fix the behaviour for the --generate-only flag

[netroy]

Settings stores the flag in camelCase.
But, the flag is currently read in kebab-case.

This PR fixes the bug, & adds some tests for expected behavior.

Fixes #5030

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[mergify]

Thanks so much for taking the time to contribute to the AWS CDK ❤️

We will shortly assign someone to review this pull request and help get it
merged. In the meantime, please take a minute to make sure you follow this
checklist:

• PR title type(scope): text
  • type: fix, feat, refactor go into CHANGELOG, chore is hidden
  • scope: name of module without aws- or cdk- prefix or postfix (e.g. s3 instead of aws-s3-deployment)
  • text: use all lower-case, do not end with a period, do not include issue refs
• PR Description
  • Rationale: describe rationale of change and approach taken
  • Issues: indicate issues fixed via: fixes #xxx or closes #xxx
  • Breaking?: last paragraph: BREAKING CHANGE: <describe what changed + link for details>
• Testing
  • Unit test added. Prefer to add a new test rather than modify existing tests
  • CLI or init templates change? Re-run/add CLI integration tests
• Documentation
  • README: update module README to describe new features
  • API docs: public APIs must be documented. Copy from official AWS docs when possible
  • Design: for significant features, follow design process

Pull request has been modified.

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[shivlaks]

just to confirm one of the PR checklist items did you also re-run the integ tests and verify they all pass?

[netroy]

just to confirm one of the PR checklist items did you also re-run the integ tests and verify they all pass?

@shivlaks I did.

[mergify]

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