test(nextjs): Make tracingFetch integration test not use UI elements

[lforst]

Ref: #6281

This PR attempts to fix a flakey Next.js integration test that seems to be flakey because the UI thread was blocking a button-click interaction, causing the page load transaction to terminate before we get the chance to trigger a fetch request via the button click.

We try to fix this by conducting the fetch request on page mount, so we decouple the request from a UI interaction.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.5 KB (-0.02% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	60.29 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.16 KB (+0.01% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	53.63 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	19.9 KB (0%)
@sentry/browser - Webpack (minified)	65.12 KB (0%)
@sentry/react - Webpack (gzipped + minified)	19.92 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	45.93 KB (0%)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	26.37 KB (0%)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	24.78 KB (+0.02% 🔺)