Add additional unit tests to fleet-server config

[michel-laterman]

Follow up to #2495 to add tests for additional coverage

[mergify]

This pull request does not have a backport label. Could you fix it @michel-laterman? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 8./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[elasticmachine]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Start Time: 2023-04-13T15:03:54.455+0000

• Duration: 17 min 36 sec

Test stats 🧪

Test	Results
Failed	0
Passed	5515
Skipped	19
Total	5534

💚 Flaky test report

Tests succeeded.

🤖 GitHub comments

Expand to view the GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• /package : Generate the packages.

• run integration tests : Run the Elastic Agent Integration tests.

• run end-to-end tests : Generate the packages and run the E2E Tests.

• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)

[elasticmachine]

🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	98.529% (67/68)	👍 0.022
Files	68.22% (161/236)	👎 -1.649
Classes	67.785% (303/447)	👎 -1.322
Methods	54.052% (927/1715)	👎 -0.322
Lines	39.602% (10497/26506)	👎 -0.005
Conditionals	100.0% (0/0)	💚

[blakerouse]

Thanks for the additional tests!