TST: Move `test_crosstab_margins` to `TestPivotTable`
This test case assert `pivot_table` method.
So it should be defined on `TestPivotTable`.

@yui-knk Seems reasonable indeed.

There is one failing test, but you don't need to bother about that (it's data-reader related, they fail once in a while), but there is also a lint error. You just need to add an extra blank line after the test function (pep8). Can you update for that?
 lgtm (assume pep is fixed)
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13553?src=pr) is **84.34%**

> Merging [#13553](https://codecov.io/gh/pydata/pandas/pull/13553?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #13553   diff @@
==========================================
  Files           138        138          
  Lines         51126      51126          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43122      43122          
  Misses         8004       8004          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [7c9ba14...e996e6a](https://codecov.io/gh/pydata/pandas/compare/7c9ba14c6ddd91916ffd7fc5548731265533e88d...e996e6a8c883f54d1817a62ca620e358675103f9)
