RSpec test ids are relative to the working directory, not SimpleCov.root #1322

track_tests builds an RSpec example's id from RSpec's location metadata (and, since #1319, example.id), which RSpec makes relative to the directory rspec runs from. Everything else treats test ids as relative to SimpleCov.root:

• Minitest ids strip SimpleCov.root from the test's source_location.
• TestTracker#test_file_loading records a spec file's loading relative to SimpleCov.root.
• Contexts#deleted_test? checks whether a test still exists with File.expand_path(file, SimpleCov.root).
• simplecov affected checks test paths and starts --run at the root.

When rspec runs from a directory other than SimpleCov.root, say engines/foo with the root at the repository root, the example is recorded as spec/b_spec.rb:5 while its file's loading is recorded as engines/foo/spec/b_spec.rb. Then:

• One spec file gets two unrelated contexts, so at granularity: :file its loading and its examples are never merged.
• Contexts#deleted_test? finds no spec/b_spec.rb under the root and drops the example from the carried-forward map on every merge.
• simplecov affected --run passes spec/b_spec.rb:5 to rspec started at the root, where that path does not exist.

A spec file outside the working directory is worse, since RSpec leaves its path absolute.

The fix is probably to build RSpec ids from the absolute path (absolute_file_path) relative to SimpleCov.root, the way Minitest ids are built, in the one place that already normalizes them, TestTracker.rspec_path (added in #1320).

CC: @meganemura