fix(tests): Update loader integration tests to avoid flakes.

[onurtemizkan]

Fixes: #8590

Playwright's event listeners and page.goto functions can occasionally end up in race condition, even when they are invoked in the correct order.

The workaround is to invoke them in Promise.all, unless there's a specific need to separate them.

This PR updates loader tests to use that pattern.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	21.95 KB (-0.01% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	69.13 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	20.28 KB (-0.01% 🔽)
@sentry/browser - ES6 CDN Bundle (minified)	60.38 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	21.91 KB (0%)
@sentry/browser - Webpack (minified)	71.51 KB (0%)
@sentry/react - Webpack (gzipped + minified)	21.92 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	50.76 KB (0%)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	30.33 KB (-0.01% 🔽)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	28.2 KB (-0.01% 🔽)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	49.56 KB (+0.25% 🔺)
@sentry/replay - Webpack (gzipped + minified)	43.33 KB (+0.4% 🔺)
@sentry/browser + @sentry/tracing + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	69.68 KB (+0.17% 🔺)
@sentry/browser + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	61.96 KB (+0.21% 🔺)

[mydea]

makes sense to me, thank you!

[Lms24]

Thanks Onur! Looks good to me.