old_contents: export default {
    input: 'tests/**/*-test.js',
    external: ['ava'],
    plugins: [],
  
    output: {
      file: 'build/test-bundle.js',
      format: 'cjs',
      sourcemap: true
    }
  };
  
new_contents: export default {
    input: 'tests/simple-test.js',
    external: ['ava'],
    plugins: [],
  
    output: {
      file: 'build/test-bundle.js',
      format: 'cjs',
      sourcemap: true
    }
  };
  
subject: use single test file as input