fix web-ext test

test.program.js

  it('throws an error about unknown sub-command options', () => {
    const program = new Program(['thing', '--nope'])
      .command('thing', '', () => {});
    return run(program)
      .then(makeSureItFails())
      .catch((error) => {
        // Again, yargs calls this an argument not an option for some reason.
        assert.match(error.message, /Unknown argument: --nope/);
      });
  });