TST: Add interval closed fixture to top-level conftest
Noticed identical copies of this fixture in multiple places, so seemed reasonable to dedupe and move to the top-level `conftest.py`.  I think I removed all instances of such a fixture/parametrize being used, but could have missed one.  I imagine this fixture will be used in more places in the future when interval related tests are expanded.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21595?src=pr&el=h1) Report
> Merging [#21595](https://codecov.io/gh/pandas-dev/pandas/pull/21595?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/028c9c0ef78878c9ef01570b47dd5211ef9d26ca?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/21595/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/21595?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21595   +/-   ##
=======================================
  Coverage    91.9%    91.9%           
=======================================
  Files         153      153           
  Lines       49549    49549           
=======================================
  Hits        45539    45539           
  Misses       4010     4010
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <ø> (ø)` | :arrow_up: |
| #single | `41.78% <ø> (ø)` | :arrow_up: |


------

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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21595?src=pr&el=footer). Last update [028c9c0...c1f84a2](https://codecov.io/gh/pandas-dev/pandas/pull/21595?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks. its possible this *might* conflict with 'closed' as used in thinks like ``pd.date_range`` and ``resample`` which only have ``left/right`` but no big deal.