ci: Improve flaky test detector performance

[mydea]

We now run all tests into a single spawned process, which should hopefully speed this up a bit. We also reduce the # of repetitions from 100 to 50 to ensure we do not run overtime.

I noticed this is timing out on CI if there are a few changed tests as of now.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	20.44 KB (-0.01% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	63.52 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	19.13 KB (0%)
@sentry/browser - ES6 CDN Bundle (minified)	56.57 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	21.59 KB (0%)
@sentry/browser - Webpack (minified)	71.65 KB (+0.01% 🔺)
@sentry/react - Webpack (gzipped + minified)	21.61 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	51.85 KB (-0.21% 🔽)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	27.77 KB (+0.05% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	26.08 KB (+0.06% 🔺)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	44.21 KB (0%)
@sentry/replay - Webpack (gzipped + minified)	38.28 KB (0%)
@sentry/browser + @sentry/tracing + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	62.4 KB (+0.02% 🔺)
@sentry/browser + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	55.95 KB (0%)