test for RecommenderXBlock

Nitpick: you don't actually need the `+` signs. Python with automatically concatenate strings that have no operator between them, and the parentheses make it work with the linebreaks. So you can do this:
```python
{
  "url": (
    "https://courses.edx.org/courses/MITx/3.091X/"
    "2013_Fall/courseware/SP13_Week_4/"
    "SP13_Periodic_Trends_and_Bonding/"
  )
}
```