test(e2e): Assert that there are no funky build time warnings when building Next.js apps

[lforst]

Adds a simple assertion for the build output of the nextjs E2E tests that checks if there are any funky compilation warnings.

This PR was brought to you by #9394

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	65.42 KB (0%)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	55.63 KB (0%)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	30.98 KB (0%)
@sentry/browser - Webpack (gzipped)	21.3 KB (0%)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	61.95 KB (0%)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	29.1 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped)	21.24 KB (0%)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	195.42 KB (0%)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	88.32 KB (0%)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	63.3 KB (0%)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	31.81 KB (0%)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	65.77 KB (0%)
@sentry/react - Webpack (gzipped)	21.34 KB (0%)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	82.49 KB (0%)
@sentry/nextjs Client - Webpack (gzipped)	48.13 KB (0%)
@sentry-internal/feedback - Webpack (gzipped)	16 KB (0%)

[lforst]

wat Next.js is throwing a funky build time warning by itself 🙃