1. yarn
2. yarn jest --config singular.config.js
3. yarn jest --config plural.config.js

The singular config with testPathPattern logs this warning:

● Deprecation Warning:

  Option "testPathPattern" was replaced by "testPathPatterns".

  Please update your configuration.

  Configuration Documentation:
  https://jestjs.io/docs/configuration

● Deprecation Warning:

  Option "testPathPattern" was replaced by "testPathPatterns".

  Please update your configuration.

  Configuration Documentation:
  https://jestjs.io/docs/configuration

The plural config with testPathPatterns logs this warning:

● Validation Warning:

  Unknown option "testPathPatterns" with value [/\.test\.js$/] was found.
  This is probably a typing mistake. Fixing it will remove this message.

  Configuration Documentation:
  https://jestjs.io/docs/configuration

● Validation Warning:

  Unknown option "testPathPatterns" with value [/\.test\.js$/] was found.
  This is probably a typing mistake. Fixing it will remove this message.

  Configuration Documentation:
  https://jestjs.io/docs/configuration

Both configs run the helper file.

The --testPathPatterns CLI flag works without any validation warnings and it does not run the helper file:

yarn jest --testPathPatterns=".*\.test\.js$"