Add test for the `ProcessMonitor` widget.

[codecov[bot]]

Codecov Report

Patch coverage: 82.14% and project coverage change: +0.03 :tada:

Comparison is base (d339b75) 79.32% compared to head (9a61190) 79.35%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #478 +/- ## ========================================== + Coverage 79.32% 79.35% +0.03% ========================================== Files 27 27 Lines 3685 3706 +21 ========================================== + Hits 2923 2941 +18 - Misses 762 765 +3 ``` | Flag | Coverage Δ | | |---|---|---| | python-3.10 | `?` | | | python-3.8 | `79.35% <82.14%> (-0.01%)` | :arrow_down: | | python-3.9 | `79.35% <82.14%> (-0.01%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidalab#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://codecov.io/gh/aiidalab/aiidalab-widgets-base/pull/478?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidalab) | Coverage Δ | | |---|---|---| | [tests/test\_process.py](https://codecov.io/gh/aiidalab/aiidalab-widgets-base/pull/478?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidalab#diff-dGVzdHMvdGVzdF9wcm9jZXNzLnB5) | `95.90% <82.14%> (-4.10%)` | :arrow_down: | ... and [3 files with indirect coverage changes](https://codecov.io/gh/aiidalab/aiidalab-widgets-base/pull/478/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidalab)

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