TST: Avoid global state in matplotlib tests
Replaces most uses of implicit global state from matplotlib in
test_datetimelike.py and test_series.py. This was potentially causing random failures
where a figure expected to be on a new, blank figure would instead
plot on an existing axes (that's the guess at least).

Closes #16371 (hopefully).
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16539?src=pr&el=h1) Report
> Merging [#16539](https://codecov.io/gh/pandas-dev/pandas/pull/16539?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/e60dc4cff2c6e8a2283fbb906faeb8cb01df37ff?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/16539/graphs/tree.svg?token=eZ4WkYLtcO&width=650&src=pr&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/16539?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #16539   +/-   ##
=======================================
  Coverage   90.79%   90.79%           
=======================================
  Files         161      161           
  Lines       51063    51063           
=======================================
  Hits        46365    46365           
  Misses       4698     4698
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.63% <ø> (ø)` | :arrow_up: |
| #single | `40.15% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16539?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/16539?src=pr&el=footer). Last update [e60dc4c...2c492fa](https://codecov.io/gh/pandas-dev/pandas/pull/16539?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 I'm going to rerun the occasionally failing worker 4-5 times to see if I can get a failure.

5 / 5 so far. looks much cleaner to me!