Fix group priority

[creativeprojects]

• Fix a bug when setting the process priority on Linux, macOS and unixes in general
• Refactoring to avoid duplicate code
• should also work inside a container

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 64.70588% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 70.85%. Comparing base (4255868) to head (893cf51). Report is 1 commits behind head on master.

Files	Patch %	Lines
priority/unix.go	73.33%	3 Missing and 1 partial :warning:
priority/ioprio.go	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #339 +/- ## ========================================== + Coverage 70.77% 70.85% +0.09% ========================================== Files 120 121 +1 Lines 12294 12273 -21 ========================================== - Hits 8700 8696 -4 + Misses 3197 3183 -14 + Partials 397 394 -3 ``` | [Flag](https://app.codecov.io/gh/creativeprojects/resticprofile/pull/339/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fred) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/creativeprojects/resticprofile/pull/339/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fred) | `70.85% <64.71%> (+0.09%)` | :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=Fred#carryforward-flags-in-the-pull-request-comment) to find out more.

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[creativeprojects]

• [x] refactoring to remove duplicates
• [x] cleanup tests
• [x] run tests inside a goroutine (it doesn't seem to make any difference)
• [x] use default process group
• [x] creates own process group only if current group fails: should fix priority inside containers
• [x] verified restic is running with expected priority