Add test for DBus RuleEnable and fix memory leak

[zappolowski]

This is mainly done to test whether the current pipeline works properly in terms of coverage reporting.

A nice side-effect is, that I could increase the test coverage a bit and fixed a memory leak while doing so.

[codecov-commenter]

Codecov Report

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

Project coverage is 65.98%. Comparing base (07b68f0) to head (38b794c). Report is 3 commits behind head on master.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1328 +/- ## ========================================== + Coverage 65.71% 65.98% +0.27% ========================================== Files 50 50 Lines 8041 8056 +15 Branches 925 925 ========================================== + Hits 5284 5316 +32 + Misses 2757 2740 -17 ``` | [Flag](https://app.codecov.io/gh/dunst-project/dunst/pull/1328/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dunst-project) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/dunst-project/dunst/pull/1328/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dunst-project) | `65.98% <100.00%> (+0.27%)` | :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=dunst-project#carryforward-flags-in-the-pull-request-comment) to find out more.

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[bynect]

can I merge it ?