Added parenthesis to fix the structure_remove_begin_1 testcase

Hi,

The `structure_remove_begin_1 testcase` testcase in `lib/pt_testcase.rb` is incorrect. Expected code for the supplied sexp is `(a << b rescue c)` when it should be `(a << (b rescue c))`.

Here is the sexp used in that testcase (line 2851):
```
s(:call, s(:call, nil, :a), :<<,
  s(:rescue,
    s(:call, nil, :b),
    s(:resbody, s(:array),
      s(:call, nil, :c))))
```

Notice how the :rescue sexp wraps the argument of the `:<<` call. The problem is that the expected code `(a << b rescue c)` will try to rescue from `a << b` instead of just `b` because of operator precedence. Indeed, looking at the sexp corresponding to the current (incorrect) expected code:
```
RubyParser.new.process "(a << b rescue c)"
```
returns
```
s(:rescue,
  s(:call, s(:call, nil, :a), :<<, s(:call, nil, :b)),
  s(:resbody, s(:array), s(:call, nil, :c)))
```

Notice how the rescue sexp is outside the call this time. This is the correct semantic of the faulty expected code.

Therefore, the expected code should be changed to `(a << (b rescue c))`.

Thanks,
- Bocete