test(e2e): Assert correct isolation scopes in Next.js

[lforst]

Add assertions to the Next.js SDK tests that the relevant spans/transactions do have tags when Sentry.setTag() is called inside their handlers, which in turn tests whether we write to the correct isolation scope and that we are not leaking data between handlers.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser	22.1 KB (0%)
@sentry/browser (incl. Tracing)	31.71 KB (0%)
@sentry/browser (incl. Tracing, Replay)	66.92 KB (0%)
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags	60.52 KB (0%)
@sentry/browser (incl. Tracing, Replay with Canvas)	70.75 KB (0%)
@sentry/browser (incl. Tracing, Replay, Feedback)	75.67 KB (0%)
@sentry/browser (incl. Feedback)	30.87 KB (0%)
@sentry/browser (incl. Feedback, Feedback Modal)	30.89 KB (0%)
@sentry/browser (incl. Feedback, Feedback Modal, Feedback Screenshot)	30.89 KB (0%)
@sentry/browser (incl. sendFeedback)	26.88 KB (0%)
@sentry/react	24.78 KB (0%)
@sentry/react (incl. Tracing)	34.61 KB (0%)
@sentry/vue	25.53 KB (0%)
@sentry/vue (incl. Tracing)	33.44 KB (0%)
@sentry/svelte	22.23 KB (0%)
CDN Bundle	24.11 KB (0%)
CDN Bundle (incl. Tracing)	32.66 KB (0%)
CDN Bundle (incl. Tracing, Replay)	66.37 KB (0%)
CDN Bundle (incl. Tracing, Replay, Feedback)	71.63 KB (0%)
CDN Bundle - uncompressed	71.67 KB (0%)
CDN Bundle (incl. Tracing) - uncompressed	97.71 KB (0%)
CDN Bundle (incl. Tracing, Replay) - uncompressed	207.1 KB (0%)
@sentry/nextjs (client)	33.8 KB (0%)
@sentry/sveltekit (client)	32.24 KB (0%)
@sentry/node	119.95 KB (0%)