TST: Fix MMapWrapper init test for Windows
Turns out Windows errors differently when an invalid `fileno` is passed into the `mmap` constructor, so there's no need to skip the test (xref: 9670b31).

lgtm, ping when green.
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13494?src=pr) is **84.33%**

> Merging [#13494](https://codecov.io/gh/pydata/pandas/pull/13494?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #13494   diff @@
==========================================
  Files           138        138          
  Lines         51106      51106          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43102      43102          
  Misses         8004       8004          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [1a12ead...db7c6b1](https://codecov.io/gh/pydata/pandas/compare/1a12eadaf92eb1ee05446d24e5d7a12cde971ce3...db7c6b1190f3e173dda5e0097bc5e65a835c0d90)
 @jreback : Travis is happy, so ready to merge if there are no concerns.
