fix(core): Check that functions have been called in beforeSend and beforeSendTransaction tests

[lobsterkatie]

Per @Lms24's suggestion, this adds a check to make sure the relevant function is being called in our beforeSend and beforeSendTransaction tests.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.5 KB (+0.06% 🔺)
@sentry/browser - ES5 CDN Bundle (minified)	60.37 KB (+0.04% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.15 KB (+0.19% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	53.71 KB (+0.07% 🔺)
@sentry/browser - Webpack (gzipped + minified)	19.89 KB (+0.17% 🔺)
@sentry/browser - Webpack (minified)	65.12 KB (-0.01% 🔽)
@sentry/react - Webpack (gzipped + minified)	19.92 KB (+0.19% 🔺)
@sentry/nextjs Client - Webpack (gzipped + minified)	45.82 KB (+0.06% 🔺)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	26.28 KB (+0.06% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	24.67 KB (+0.12% 🔺)