test(tracing): Add tests for pageload spans.

[onurtemizkan]

Part of: #4262

Tests checking the existence of browser-related and resource-related spans inside pageload transactions.

Note: connect, cache and DNS spans are never created on Firefox, and occasionally not created on Webkit and Chromium.

[github-actions]

size-limit report

Path	Base Size (e52faa4)	Current Size	Change
@sentry/browser - CDN Bundle (gzipped)	19.77 KB	19.77 KB	+0.02% 🔺
@sentry/browser - CDN Bundle (minified)	62.94 KB	62.94 KB	+0.01% 🔺
@sentry/browser - Webpack	22.31 KB	22.31 KB	0%
@sentry/browser - Webpack - gzip = false	76.19 KB	76.19 KB	0%
@sentry/react - Webpack	22.34 KB	22.34 KB	0%
@sentry/nextjs Client - Webpack	46.48 KB	46.48 KB	0%
@sentry/browser + @sentry/tracing - CDN Bundle (gzipped)	28.43 KB	28.43 KB	+0.01% 🔺

[AbhiPrasad]

awesome work!