Refactor backtrace formatter implementation.

- doesnt_match_inclusion_pattern_unless_system_exclusion?
  has a double negative and was being used in a compound
  conditional, which made my head explode. I couldn't
  understand it at all.
- Reuse `Metadata.relative_path` rather than having our own
  logic to do that.
- It'll perform better to get the relative path once rather
  than doing it once per pattern in the `patterns.any?` block.
- Add spec for case that is covered by a cucumber scenario but
  not by an existing spec (an earlier version of this commit
  broke that cucumber scenario but no specs failed, revealing
  the missing spec coverage).