testEntryObjectOneType(type) {
    const keys = Object.keys(this.rule);
    for (let i = 0; i < keys.length; i += 1) {
      const key = keys[i];
      if (!Rule.TEST_FUNCTIONS[type]) {
        throw Error(`The \`${type}\` type doesn't exist`);
      }
      if (!Rule.TEST_FUNCTIONS[type][key]) {
        throw new Error(`\`${type}\` doesn't have "${key}" test!`);
      }
    }
  }
Tests the validity of the constructor object
thows an error if the object is invalid
tests if all the keys are valid