Simplify coverage tests

This change reduces the risk of impacting coverage tests on unrelated
changes (such as MIR and Span changes), and reduces the burden when
blessing coverage changes in case it is necessary.

* Remove all spanview tests. The spanview tests were useful during
  development, but they can be generated as needed, via compiler command
  line flags. They aren't critical to confirming coverage results. (The
  coverage report tests are sufficient.)

  When spanview regeneration was necessary, the diffs were way too hard
  to read to be useful anyway. So I'm removing them to reduce friction
  from a feature that is no longer useful.

* Remove the requirement for `llvm-cov show --debug` when blessing
  tests. The `--debug` flag is, unfortunately, only available if LLVM is
  built with `optimize = false` (in Rust's config.toml). This adds
  significant time and resource burdens to the contributor's build. As
  it turns out, for other reasons in the past, I wasn't actually using
  the debug output (counter info) to validate coverage anymore either,
  so it was required for no reason, I now realize.

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