Fix race detector errors in TestCoordinatorDiagnosticHooks

[faec]

TestCoordinatorDiagnosticHooks triggered the race detector because it accessed bare variables in its mocks from multiple goroutines. This fix makes the relevant conditions atomic and sets them from explicit callbacks instead. Followup from #2790.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool
• I have added an integration test or an E2E test

[mergify]

This pull request does not have a backport label. Could you fix it @faec? 🙏
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]

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• Start Time: 2023-06-09T09:28:05.912+0000

• Duration: 21 min 46 sec

Test stats 🧪

Test	Results
Failed	0
Passed	5983
Skipped	19
Total	6002

💚 Flaky test report

Tests succeeded.

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[elasticmachine]

🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	98.667% (74/75)	👍
Files	68.605% (177/258)	👍
Classes	67.423% (327/485)	👍
Methods	53.698% (1002/1866)	👍
Lines	39.577% (11471/28984)	👎 -0.028
Conditionals	100.0% (0/0)	💚

[blakerouse]

Looks good.