fix(test): Unflake LCP test

[onurtemizkan]

Moved assertions inside a conditional where the lcp.element is the image, not the button (It's worked around like this in other test suites). I guess this flakiness is behavioural not about the tests.

[codecov]

❌ 4 Tests Failed:

Tests completed	Failed	Passed	Skipped
616	4	612	29

View the top 3 failed tests by shortest run time

nested-rsc-error.test.ts Should capture errors from nested server components when `Sentry.captureRequestError` is added to the `onRequestError` hook

Stack Traces | 120s run time

nested-rsc-error.test.ts:6:5 Should capture errors from nested server components when `Sentry.captureRequestError` is added to the `onRequestError` hook

streaming-rsc-error.test.ts Should capture errors for crashing streaming promises in server components when `Sentry.captureRequestError` is added to the `onRequestError` hook

Stack Traces | 120s run time

streaming-rsc-error.test.ts:6:5 Should capture errors for crashing streaming promises in server components when `Sentry.captureRequestError` is added to the `onRequestError` hook

ppr-error.test.ts should not capture React-internal errors for PPR rendering

Stack Traces | 120s run time

ppr-error.test.ts:4:5 should not capture React-internal errors for PPR rendering

To view individual test run time comparison to the main branch, go to the Test Analytics Dashboard

[Lms24]

I think this is fine for now, thanks for fixing!

Ideally, this gets more stable with #12714