fix(browser): dispose tester iframe on done

[hi-ogawa]

Description

• closes Browser mode fails with SyntaxError: Failed to construct 'WebSocket' #5382

Currently iframes are not disposed and that seems to make browser mode get stuck.
We might not want to always dispose the iframe, for example, if users are expecting to view the actual browser render (cf. #5568), but I think that can be implemented as a dedicated option (like browser.retainWindow or something) and disposing them by default for stability seems right.

Test plan

I tested a same patch on https://github.com/underoot/vitest-large-file-list-issue with 300 test files and it's working fine at least with fileParallelism: false:

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