Put output dir to tmp_path

[pnuu]

The TestSaveDatasets.test_pop_unknown_args is creating a Hungarian directory and not removing it. This PR fixes it by using tmp_path fixture.

• [x] Closes #155
• [x] Tests modified
• [x] Tests passed
• [x] Passes flake8 trollflow2

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.96%. Comparing base (98355d9) to head (f834043). Report is 8 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #224 +/- ## ======================================= Coverage 95.96% 95.96% ======================================= Files 14 14 Lines 3268 3272 +4 ======================================= + Hits 3136 3140 +4 Misses 132 132 ``` | [Flag](https://app.codecov.io/gh/pytroll/trollflow2/pull/224/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/pytroll/trollflow2/pull/224/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | `95.96% <100.00%> (+<0.01%)` | :arrow_up: | 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=pytroll#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.