Add test for #24615 (Accept PandasArray (with correct dtype) in DatetimeArray constructor)
issue #24615

I added a test for the issue 24615.
@mroeschke @jorisvandenbossche  See my comment on the issue (https://github.com/pandas-dev/pandas/issues/24615), the original code snippet from that issue changed behaviour, so this PR is actually only testing that DatetimeArray accepts instances of itself. And that is something that I suppose is already tested. @gorogoroumaru mind checking if we have a test in our codebase where DatetimeArray accepts itself?