Add related test selection

[dicnunz]

Fixes #1986.

IssueHunt bounty: https://issuehunt.io/repos/26820798/issues/1986

Summary

• add --related so positional paths can be treated as changed source files
• trace configured test files with AVA's existing @vercel/nft dependency tracer
• run directly changed test files, run dependent tests for known source files, and conservatively run all tests when a changed file is not in the dependency graph
• document the new command-line mode and cover it with fixture tests

Verification

• npx test-ava test/related-tests/test.js
• npx test-ava test/related-tests/test.js test/watch-mode/scenarios.js test/globs/test.js
• npx tsc --noEmit
• npx xo (exits 0; existing TODO warnings only)
• node --check lib/related-tests.js && node --check lib/cli.js && node --check lib/api.js && node --check test/related-tests/test.js
• git diff --check

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IssueHunt Summary

Referenced issues

This pull request has been submitted to:

• #1986: Analyze source & test dependencies to figure out which tests to run (first)

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