test: reset mqtt.bridge.built_in setting to default value

[reubenmiller]

Proposed changes

Prevent an inadvertent change of the mqtt.bridge.built_in setting at the end of a test by unsetting the value instead of setting it to an explicit value (and using an official [Teardown] hook so it is always executed regardless if the test passes or fails).

Technically the test flaky is present due to an unknown issue in the builtin bridge, however this issue will be addressed in another PR, #2911.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (general improvements like code refactoring that doesn't explicitly fix a bug or add any new functionality)
• Documentation Update (if none of the other choices apply)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Paste Link to the issue

• flaky system test: tedge_connect_test_sm_services #3022

Checklist

• I have read the CONTRIBUTING doc
• I have signed the CLA (in all commits with git commit -s)
• I ran cargo fmt as mentioned in CODING_GUIDELINES
• I used cargo clippy as mentioned in CODING_GUIDELINES
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

[github-actions]

Robot Results

✅ Passed	❌ Failed	⏭️ Skipped	Total	Pass %	⏱️ Duration
486	0	2	486	100	1h25m23.635279999s