TST: fix excel tests when openpyxl is not installed
Some tests fail when you don't have openpyxl installed, so explicitly skipping those (they need it for certain exts because they write a frame in the test)
+1 I thought I had fixed these........ Is it worth to have a build without openpyxl for this? Although it should be a build without openpyxl, but with xlrd/xlwt, to have this surface, and it's also not a bug, only a testing thing, so probably not worth it prob not worth it, but no objection if u want to change one (say 3.4_nslow) ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/15209?src=pr) is 85.98% (diff: 100%)
> Merging [#15209](https://codecov.io/gh/pandas-dev/pandas/pull/15209?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will decrease coverage by **0.01%**

```diff
@@             master     #15209   diff @@
==========================================
  Files           140        140          
  Lines         51273      51272     -1   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          44091      44085     -6   
- Misses         7182       7187     +5   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [dc40058...f1cbe8f](https://codecov.io/gh/pandas-dev/pandas/compare/dc400587a3e792d90f5db0be6caff7a4e12574c0...f1cbe8f393c413c53560eb7463f7a306bb7895eb?src=pr) thanks!