TST: avoid DST transitions when testing DateTimeIndex roundtrip
- [x] closes #24689
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24736?src=pr&el=h1) Report
> Merging [#24736](https://codecov.io/gh/pandas-dev/pandas/pull/24736?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/fdc4db25a9988a7b595a3756760d05a6c177123d?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/24736/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24736?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24736   +/-   ##
=======================================
  Coverage   92.39%   92.39%           
=======================================
  Files         166      166           
  Lines       52358    52358           
=======================================
  Hits        48374    48374           
  Misses       3984     3984
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.81% <ø> (ø)` | :arrow_up: |
| #single | `43.07% <ø> (-0.03%)` | :arrow_down: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24736?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24736?src=pr&el=footer). Last update [fdc4db2...659ca34](https://codecov.io/gh/pandas-dev/pandas/pull/24736?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24736?src=pr&el=h1) Report
> Merging [#24736](https://codecov.io/gh/pandas-dev/pandas/pull/24736?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/2626215ca5dd168ea311f117b988693cef437885?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/24736/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24736?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24736   +/-   ##
=======================================
  Coverage   92.37%   92.37%           
=======================================
  Files         166      166           
  Lines       52388    52388           
=======================================
  Hits        48396    48396           
  Misses       3992     3992
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.8% <ø> (ø)` | :arrow_up: |
| #single | `42.88% <ø> (-0.02%)` | :arrow_down: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24736?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24736?src=pr&el=footer). Last update [2626215...9a9bfe2](https://codecov.io/gh/pandas-dev/pandas/pull/24736?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Looks good. Does this test pass locally for you? Yes, passes here

On Sun, 27 Jan 2019 at 07:41, Matthew Roeschke <notifications@github.com>
wrote:

> Looks good. Does this test pass locally for you?
>
> —
> You are receiving this because you authored the thread.
> Reply to this email directly, view it on GitHub
> <https://github.com/pandas-dev/pandas/pull/24736#issuecomment-457896352>,
> or mute the thread
> <https://github.com/notifications/unsubscribe-auth/ATkI0HR1PkVkOWVmalXr0_nfxOag3P0fks5vHVgNgaJpZM4Z8VRx>
> .
>
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