test(tracing): Check if beforeNavigate does not change source

[AbhiPrasad]

Since custom is the default and the test above already checks for this, I changed the semantics of the test to make sure we were setting url properly.

Did this while validating a customer issue hence why changes here.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	20.41 KB (+0.01% 🔺)
@sentry/browser - ES5 CDN Bundle (minified)	63.3 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.99 KB (+0.02% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	56.21 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	20.65 KB (0%)
@sentry/browser - Webpack (minified)	67.44 KB (0%)
@sentry/react - Webpack (gzipped + minified)	20.68 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	52.12 KB (+0.07% 🔺)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	33.66 KB (+0.02% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	26.01 KB (+0.05% 🔺)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	43.18 KB (+0.01% 🔺)
@sentry/replay - Webpack (gzipped + minified)	37.19 KB (0%)
@sentry/browser + @sentry/tracing + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	61.31 KB (+0.02% 🔺)
@sentry/browser + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	54.39 KB (+0.01% 🔺)