Add --related-to flag to run only tests affected by changed files

[stevenmini2019]

Summary

Implements #1986 by adding a --related-to <files> CLI flag that runs only the test files which (transitively) import the given source files — the AVA equivalent of jest --findRelatedTests.

This is intended for pre-commit hooks (e.g. lint-staged) where you only want to run the tests that could be affected by the files that changed, instead of the whole suite.

npx ava --related-to src/foo.js
npx ava --related-to src/foo.js src/bar.js
npx ava --related-to 'src/**/*.js'

How it works

• A new lib/related-tests.js exposes createRelatedTestSelector(relatedTo, projectDir).
• It walks the static import graph of every discovered test file (resolving relative/absolute specifiers, following transitive imports, and skipping bare package specifiers as external dependencies).
• It builds a reverse map source file → test files and keeps only the test files connected to any of the files passed to --related-to.
• The selector is wired into lib/cli.js as the existing testFileSelector hook on Api#run, so it composes with other patterns/--match filters.

Notes / limitations

• Only relative and absolute import specifiers are followed; bare (package) imports are treated as external and do not connect a test to a project source file.
• Type-only imports (import type {…}) are followed as well, which can make the selection slightly over-inclusive but never under-inclusive.
• The selection respects any other patterns or filters passed, so --related-to can be combined with a glob or --match.

Test plan

• test/related-tests/test.js unit-tests the selector: single-file selection, multi-file selection, transitive imports, interaction with an existing selection filter, empty input, and space/comma-separated inputs.
• Verified end-to-end: ava --related-to <source> runs only the related test file(s) out of the full discovered set, including the transitive case where src/foo.js imports src/bar.js.