Add examples for one-liner syntax

RSpec's "is_expected" syntax isn't currently supported. The one-liner "should" syntax seems to be supported, but isn't included in the specs. This pull request adds specs for both one-liner styles.

- "should" syntax passes
- "is_expected" syntax fails

```
$ bundle exec rake

Failures:


  1) one-liner syntax is_expected
     NoMethodError:
       undefined method `is_expected' for #<RSpec::ExampleGroups::OneLinerSyntax::IsExpected:1325>


  2) one-liner syntax is_expected
     NoMethodError:
       undefined method `is_expected' for #<RSpec::ExampleGroups::OneLinerSyntax::IsExpected:1333>

63 examples, 2 failures (time taken: 1.101)
```

I'd love to add this feature, but I'm having trouble tracing the root issue. The `is_expected` method seems to be defined [in the same file](https://github.com/rspec/rspec-core/blob/2-99-maintenance/lib/rspec/core/memoized_helpers.rb) as `should`.

Any advice on how to approach this?