[ruby/irb] Improve RubyLex's tests

(https://github.com/ruby/irb/pull/484)

* Improve assert_indenting helper

Instead of putting assertions inside the `auto_indent` block, we
can just make `auto_indent` return the calculated space count, and use
it for assertion outside of the `auto_indent` block call.

This simplifies the setup code and makes the intention easier to
understand.

* Introduce assert_row_indenting helper

1. Helper users shouldn't need to write 2 assertions for the current and
   the next line's indentation.
2. With this new approach, we can generate clearer error message for
   both cases:

When the current line's space count doesn't match

```
  Incorrect spaces calculation for line:

  ```
> def each_top_level_statement
  ```

  All lines:

  ```
  def each_top_level_statement
  ```

<0> expected but was
<nil>
```

When the next line's space count doesn't match

```
  Incorrect spaces calculation for line after the current line:

  ```
  def each_top_level_statement
>
  ```

  All lines:

  ```
  def each_top_level_statement
  ```

<3> expected but was
<2>
```

* Replace assert_indenting with assert_row_indenting

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