test(replay): Improve replay test mocks

[mydea]

This PR does a few internal improvements to the replay tests.

Basically, there is a bit of a mix and match of how/when modules are mocked in the replay tests, making it hard to refactor some things. A bunch of things depend on import order etc. which is not great.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.54 KB (-0.01% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	60.42 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.21 KB (+0.02% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	53.77 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	19.94 KB (0%)
@sentry/browser - Webpack (minified)	65.25 KB (0%)
@sentry/react - Webpack (gzipped + minified)	19.96 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	45.96 KB (0%)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	26.41 KB (-0.01% 🔽)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	24.83 KB (+0.01% 🔺)
@sentry/replay index.js	83.65 KB (-0.07% 🔽)
@sentry/replay - Webpack (gzipped + minified)	83.67 KB (-0.07% 🔽)
@sentry/replay - Webpack (minified)	272.53 KB (-0.06% 🔽)