TST: add comprehensive coercion tests
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`

1st attempt to make dtype related tests more comprehensive. 

Validate whether tests classes have test methods covering dtype and Series/Index. Some of them are empty yet, going to add it gradually. 

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14010?src=pr) is 85.29% (diff: 100%)

> Merging [#14010](https://codecov.io/gh/pydata/pandas/pull/14010?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #14010   diff @@
==========================================
  Files           139        139          
  Lines         50241      50241          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          42849      42851     +2   
+ Misses         7392       7390     -2   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [6b7857b...de3eb22](https://codecov.io/gh/pydata/pandas/compare/6b7857b0154353f3fc1e80110876371cb10b2136...de3eb22b8ccbf720f9d56891250ffda2b0ca5d34?src=pr)
 thanks @sinhrks you are great on these comprehensive tests!
