Reads a do / while loop
```ebnf
do ::= T_DO statement T_WHILE '(' expr ')' ';'
```
@see https://github.com/php/php-src/blob/master/Zend/zend_language_parser.y#L423
@return {Do}
function() {
    const result = this.node("do");
    this.expect(this.tok.T_DO) && this.next();
    let test = null;
    let body = null;
    body = this.read_statement();
    if (this.expect(this.tok.T_WHILE)) {
      if (this.next().expect("(")) this.next();
      test = this.read_expr();
      if (this.expect(")")) this.next();
      if (this.expect(";")) this.next();
    }
    return result(test, body);
  }