test: update smoketest to run elastic-agent in unprivileged mode

[kruskall]

Motivation/summary

Update smoketest to run elastic-agent in unprivileged mode. This ensure we have testing coverage for unprivileged mode and everything works correctly.

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

EC_API_KEY=$(<apikey>) make smoketest/run TEST_DIR=testing/smoke/managed/

Related issues

Closes #12503

[mergify]

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

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./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.