fix(tests): Increase flush interval of session tests.

[onurtemizkan]

Resolves: #6868

Increasing flush interval to 1000ms to 2000ms, with overall timeout from 2000ms to 4000ms.

Looks like the test flakes because of early-flushing (before 'slow' endpoint finishes), so also decreased the slowness of the 'slow' endpoint.

This branch consistently succeeded on local, will test on CI here.

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Update: None of the 11 CI runs here have failed session tests.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.83 KB (-0.01% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	61.48 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.5 KB (-0.01% 🔽)
@sentry/browser - ES6 CDN Bundle (minified)	54.79 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	20.22 KB (0%)
@sentry/browser - Webpack (minified)	66.19 KB (0%)
@sentry/react - Webpack (gzipped + minified)	20.25 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	47.48 KB (0%)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	26.75 KB (-0.01% 🔽)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	25.03 KB (-0.01% 🔽)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	43.26 KB (-0.01% 🔽)
@sentry/replay - Webpack (gzipped + minified)	38.04 KB (0%)
@sentry/browser + @sentry/tracing + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	60.52 KB (-0.01% 🔽)