Add file URL support for test runners

[JLHwung]

Q	A
Fixed Issues?	Fixes #1, Fixes #2
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

In this PR we add file URL support for test runners and phase out usage of __dirname in the test files.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/61564

[Copilot]

Pull request overview

This PR adds file URL support to Babel’s fixture/test runners and updates tests to avoid __dirname-based fixture resolution where possible.

Changes:

• Allows fixture helpers and transform/process test runners to accept URL instances.
• Updates parser, generator, CLI/node helper, and ESLint parser tests to pass fixture locations as file URLs.
• Moves the SystemJS exec-specific test into its own Jest test file using URL-based filename handling.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/babel-helper-fixtures/src/index.ts	Accepts URL fixture roots by converting them to filesystem paths.
packages/babel-helper-transform-fixture-test-runner/src/index.ts	Accepts URL filenames/fixture dirs for transform and process runners.
packages/babel-helper-plugin-test-runner/src/index.ts	Passes fixture roots as URLs and derives package names via fileURLToPath.
packages/babel-parser/test/index.js	Uses a file URL for parser fixtures.
packages/babel-parser/test/expressions.js	Uses a file URL for expression fixtures.
packages/babel-types/test/fields.js	Uses a file URL for parser fixture discovery.
packages/babel-generator/test/index.js	Uses a file URL for generator fixtures.
packages/babel-cli/test/helpers/runner.js	Passes CLI fixtures as a file URL.
packages/babel-node/test/helpers/runner.js	Passes node fixtures as a file URL.
packages/babel-build-external-helpers/test/helpers/runner.js	Passes external-helper fixtures as a file URL.
packages/babel-plugin-transform-modules-systemjs/test/index.js	Leaves the generic fixture runner entrypoint only.
packages/babel-plugin-transform-modules-systemjs/test/systemjs-exec.test.js	Adds the extracted SystemJS exec regression test using URL filenames.
eslint/babel-eslint-parser/test/typescript-estree.test.js	Uses URL-based parser fixture roots and URL href comparisons for known failures.

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