test: enable race detector

[kruskall]

Motivation/summary

Unfortunately we can only enable the race detector for ci and unit tests.
system-tests are building the apm-server binary (CGO disabled by default) so we can't enable the race detector as it requires cgo.

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

Related issues

[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.

[marclop]

There's a test that should probably be fixed, seems like a sync problem.

[kruskall]

This uncovered a race condition in elastic-agent-client lib: elastic/elastic-agent-client#125

PR blocked until that's fixed