Run tests for the React codebase. Arguments: - $ARGUMENTS: Channel, flags, and test pattern Usage Examples: - - Run with source channel (default) - - Run with experimental channel - - Run with www-modern channel - - Test VARIANT =false - - Run with stable channel - - Run with www-classic channel - - Run in watch mode (TDD) Release Channels: - - Source/canary channel, uses ReactFeatureFlags.js defaults - - Source/experimental channel with EXPERIMENTAL flags = true - - www-modern channel with VARIANT flags = true - - www channel with VARIANT flags = false - - What ships to npm - - Legacy www-cl…