test(tracing): Add TTFB tests.

[onurtemizkan]

Part of: #4262

Only checks the existence of measurement entries, as I could not find a way to emulate TTFB values like I've done with connection.rtt (#4423). Suggestions are welcome.

[github-actions]

size-limit report

Path	Base Size (eaf2e81)	Current Size	Change
@sentry/browser - CDN Bundle (gzipped)	19.59 KB	19.59 KB	+0.02% 🔺
@sentry/browser - CDN Bundle (minified)	62.5 KB	62.5 KB	+0.01% 🔺
@sentry/browser - Webpack	22.11 KB	22.11 KB	0%
@sentry/browser - Webpack - gzip = false	75.59 KB	75.59 KB	0%
@sentry/react - Webpack	22.14 KB	22.14 KB	0%
@sentry/nextjs Client - Webpack	46.33 KB	46.33 KB	0%
@sentry/browser + @sentry/tracing - CDN Bundle (gzipped)	28.3 KB	28.31 KB	+0.02% 🔺

[AbhiPrasad]

I think this is fine for now as a sanity check