(es/testing) Test function panics if existing output file has invalid code when UPDATE=1

[leafypout]

Describe the feature

When running something like UPDATE=1 cargo test, if an output file already exists for the test, say test.js, and test.js contains invalid code, e.g. a = <invalid>;, the test function will panic when trying to parse the expected code. The error should probably be silently ignored if UPDATE=1.

Babel plugin or link to the feature description

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Additional context

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