test: add test bundler compatibility

[ChALkeR]

1. const __dirname renamed to const _dirname to avoid conflict, as bundler tries to replace __dirname variables
2. process.env is undefined, replaced to globalThis.process?.env?. in one place
3. added a hint in comment for CROSS_PATH_GZ so that bundler can locate that file and include it
4. fallback to guarded require() in json() loader, as bundler is not aware of those files, but can glob require()
5. renamed test/index.test.js to test/index.js -- it's not a test itself, and we don't want test runner that loads *.test.js files to load all tests twice

To test:

• npm run lint && npm test
• CI
• npx @exodus/test (Node.js)
  This is also faster than npm test here 🙃
• npx @exodus/test --engine jsc:bundle --bundle-entropy-size=1000000 --testTimeout 5000000 (JSC)
  Requires jsc available in PATH
• npx @exodus/test --engine hermes:bundle --bundle-entropy-size=1000000 --testTimeout 5000000 (Hermes)
  Requires hermes available in PATH or https://npmjs.com/hermes-engine-cli installed with @exodus/test:
  npm add @exodus/test hermes-engine-cli