TST: Test datetime array assignment with different units (#7492)
 - [x] closes #7492
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``

The example in this issue currently works in master. Added a test to confirm. Based on a quick skim of the recent PRs, doesn't look like it was fixed recently. 

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14884?src=pr) is 85.30% (diff: 100%)


> No coverage report found for **master** at 3ba2cff.

> Powered by [Codecov](https://codecov.io?src=pr). Last update [3ba2cff...56fdc1f](https://codecov.io/gh/pandas-dev/pandas/compare/3ba2cff9c55cd16b172f9feb09da551990753f3b...56fdc1f4b954da0573d4aa3bc5335971a12e603d?src=pr) Changes made and all green.  @mroeschke Thanks!