TST: Add first line comment tests in read_csv
Title is self-explanatory.  Closes #4623.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13881?src=pr) is 85.27% (diff: 100%)

> Merging [#13881](https://codecov.io/gh/pydata/pandas/pull/13881?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **0.05%**

``` diff
@@             master     #13881   diff @@
==========================================
  Files           140        139     -1   
  Lines         50400      50011   -389   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          42951      42649   -302   
+ Misses         7449       7362    -87   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [1f55e91...becaff7](https://codecov.io/gh/pydata/pandas/compare/1f55e919a3b85cd0891a7a7a1d6913ca06f6bd9e...becaff79cd1d6dcbb73e9c6402d9433471b652a1?src=pr)
 ty sir!
