Rollup merge of #156604 - Zalathar:context-mismatch, r=mu001999

coverage: Reduce and clarify the context-mismatch test case

Most of the complexity in this test case isn't needed to reproduce the condition observed in https://github.com/rust-lang/rust/issues/147339.

I have checked that this reduced test case still triggers the context-mismatch in `extract_refined_covspans`.