TST/CLN: clean up indexes/multi/test_unique_and_duplicates
Splitting up #21645

* Added tests for `duplicated`, including a fixture for a `MultiIndex` with duplicates
* Broke up a huge test (`test_duplicates`) into smaller chunks
* removed a test (`test_unique_na`) that was for an `Index` (not `MultiIndex`), and exists verbatim in `tests/indexes/common.py`
* moved `test_duplicate_level_names_access_raises` to its appropriate module

Test failure appears unrelated.  Can you push again so it re-runs? # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21900?src=pr&el=h1) Report
> Merging [#21900](https://codecov.io/gh/pandas-dev/pandas/pull/21900?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/610a19a333c40ff67b8feb5f96c29cfae8495a3e?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21900/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/21900?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21900      +/-   ##
==========================================
+ Coverage   91.95%   91.96%   +<.01%     
==========================================
  Files         166      166              
  Lines       50274    50323      +49     
==========================================
+ Hits        46231    46281      +50     
+ Misses       4043     4042       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.36% <ø> (+0.01%)` | :arrow_up: |
| #single | `42.23% <ø> (+0.06%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21900?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/21900/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy90aW1lZGVsdGFzLnB5) | `91.53% <0%> (-0.65%)` | :arrow_down: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/21900/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.35% <0%> (-0.18%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/21900/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.16% <0%> (-0.18%)` | :arrow_down: |
| [pandas/core/dtypes/inference.py](https://codecov.io/gh/pandas-dev/pandas/pull/21900/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2luZmVyZW5jZS5weQ==) | `98.41% <0%> (ø)` | :arrow_up: |
| [pandas/core/arrays/timedelta.py](https://codecov.io/gh/pandas-dev/pandas/pull/21900/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3RpbWVkZWx0YS5weQ==) | | |
| [pandas/core/arrays/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/21900/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3RpbWVkZWx0YXMucHk=) | `89.84% <0%> (ø)` | |
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/21900/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `93.73% <0%> (ø)` | :arrow_up: |
| [pandas/core/arrays/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/21900/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lbGlrZS5weQ==) | `94.06% <0%> (+0.38%)` | :arrow_up: |
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/21900/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `97.35% <0%> (+0.41%)` | :arrow_up: |
| [pandas/core/arrays/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/21900/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lcy5weQ==) | `96.12% <0%> (+0.67%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21900?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/21900?src=pr&el=footer). Last update [610a19a...fce0d56](https://codecov.io/gh/pandas-dev/pandas/pull/21900?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jbrockmendel All green :) thanks @h-vetinari 