TST: Add unit tests for older timezone issues
- [x] closes #8616
- [x] closes #8910
- [x] closes #7825
- [x] closes #8306
- [x] closes #8531
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

In the spirit of your https://github.com/pandas-dev/pandas/pull/21407#issuecomment-396896678 @jreback, cleaned up some old timezone issues (but not necessarily covered by #21407) that appear to have been solved
@mroeschke looks awesome!

so can you either add these issue numbers to the whatsnew for #21407, or if its a 'different' issue (even though it was solved via the same), add an appropriate whatnew e.g. a resampling fixed deserves its own whatsnew (in bug fixes for all of these i think) # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21491?src=pr&el=h1) Report
> Merging [#21491](https://codecov.io/gh/pandas-dev/pandas/pull/21491?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9e982e18678c47c83ce2bb5932f844e2d9ee21f3?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21491/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/21491?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21491   +/-   ##
=======================================
  Coverage   91.92%   91.92%           
=======================================
  Files         153      153           
  Lines       49587    49587           
=======================================
  Hits        45583    45583           
  Misses       4004     4004
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.32% <ø> (ø)` | :arrow_up: |
| #single | `41.89% <ø> (ø)` | :arrow_up: |


------

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 Thanks @jreback. Yeah I don't think #21407 was directly related to any of these older issues, so made separate entries for each.  thanks @mroeschke ! it’s fine to move some to 0.24 whatsnew
though if they r indeed fixed by #21407 are ok here Sounds good. I have another timezones issues PR in the works so I'll keep that in mind
