test: add onMount test and replace onDestory test

[mcous]

Overview

This PR adds an onMount test and simplifies the onDestroy test. Supersedes #254

Change log

• Add Mounter.svelte fixture with function prop hookups to onMount and onDestroy, letting us test via spies rather than the misdirection of rendering
• Removed Stopwatch.svelte since it was a little confusing and didn't directly help the tests (see above point about misdirection)
• Removed old unmount tests because they:
  • Exercised more code than was necessary to test mount/unmount
  • Inappropriately modified the global console object without cleanup
  • Seemed to be testing implementation details (console.warn) that no longer exist

[github-actions]

🎉 This PR is included in version 4.2.0 🎉

The release is available on:

• npm package (@next dist-tag)
• GitHub release

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