Auto merge of #25220 - pshaughn:patch25151, r=jdm

Workarounds for form-submission-0 tests with about:blank onloads

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Following the model of https://github.com/servo/servo/commit/3b9ab3460029b15fc0979883fa8523443851f713 these changes replace some naive onload events with equivalents that do nothing if they see an about:blank load event. Two test cases now pass, and one failing test case is now failing for a less ambiguous reason (#25154)

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #25151 and fix #25152.

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- [X] These changes do not require tests because these are fixes to tests only, not code changes

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