Improve spec.

- The original spec had a double-slashed path like
  '/Code/gems//arbitrary'.
- Stubbing `Dir.getwd` forced the implementation to
  use that, but we'd like to reuse `RSpec::Metadata.relative_path`
  which does not use `Dir.getwd` internally.
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.