Use matplotlib's agg backend in test_figure.py.

[tjni]

By default, matplotlib will use an interactive backend when it detects that one is available. This is both unnecessary and, in some cases, can break builds.

One example of a build breaking is on macOS Ventura when these tests are run using a non-root user (such as a user used for builds) that is not the logged-in user. The interactive macOS backend crashes with an error in _xpc_api_misuse when creating an NSWindow, likely due to some tighter security requirements.

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: +0.04 :tada:

Comparison is base (d75a89b) 84.75% compared to head (751821c) 84.79%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #706 +/- ## ========================================== + Coverage 84.75% 84.79% +0.04% ========================================== Files 36 36 Lines 2105 2105 ========================================== + Hits 1784 1785 +1 + Misses 321 320 -1 ``` [see 1 file with indirect coverage changes](https://app.codecov.io/gh/lanpa/tensorboardX/pull/706/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tzu-Wei+Huang)

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[lanpa]

Thanks!