Modify testRunnerCommand to use same format as runAllTestsCommand

[ldgit]

For example, if npm t runs all our unit tests, current test runner command format would be:

module.exports = {
    {
      // ...
      arguments: ['t', '--'],      
      testRunnerCommand: 'npm',
      // ...
    },
  ],
};

New format:

module.exports = {
    {
      // ...
      testRunnerCommand: { command: 'npm', arguments: ['t', '--'] },
      // ...
    },
  ],
};

This would make it consistent with the format of runAllTestsCommand.