Reporting test span

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This could be helpful for reporting to the user or for IDEs to allow you to "run test under cursor" (if included in test discover)

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See also rust-lang/rust#116614

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In my ideal situation, we'd be reporting byte spans but proc_macro only reports file/line/column and std only provides file/line/column macros.

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Done naively, we'd report the file/line/column of the test function. However, for a parameterized test, a user may want to report the file that is driving the test operation, so the question is how should we handle both in the schema? One option is to treat this like #[track_caller] and only offer one span but allow people to override it. Maybe this would make the span a marker (#70).

Similarly, we'd like to offer the option for reporting where the test failed (which is also affected by rust code vs logical location).