Fix race conditions in ManagerV2 tests

[belimawr]

What does this PR do?

This PR fixes a number of race conditions in the ManagerV2
tests. Most of them were due to the use of Testify's Eventually
function to read some values while some callbacks from the manager
would also modify those values. The simplest solution was to use the
atomic values on those cases.

One test (TestInputReload) had a race condition between the test and
the manager itself, so it was removed. There is an integration tests
that covers the same functionality.

Why is it important?

It fixes some race conditions and #36192

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.next.asciidoc or CHANGELOG-developer.next.asciidoc.
• I have made my commit title and message explanatory about the purpose and the reason of the change

## Author's Checklist

How to test this PR locally

cd x-pack/libbeat/management
go test -race
cd ../../../
cd x-pack/filebeat
mage docker:composeUp
cd ../../
cd x-pack/filebeat/tests/integration
go test -tags=integration -race

All tests must pass and there should be no race condition.

Related issues

• Closes Build 149 for 8.8 with status FAILURE - FAIL: x-pack/libbeat/management TestInputReload (20.01s) #36192

## Use cases
## Screenshots
## Logs

[botelastic]

This pull request doesn't have a Team:<team> label.

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @belimawr? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

💚 Build Succeeded

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Build stats

• Start Time: 2023-08-21T14:34:25.363+0000

• Duration: 81 min 50 sec

Test stats 🧪

Test	Results
Failed	0
Passed	8264
Skipped	381
Total	8645

💚 Flaky test report

Tests succeeded.

🤖 GitHub comments

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[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b fix-race-condition-manager-tests upstream/fix-race-condition-manager-tests
git merge upstream/main
git push upstream fix-race-condition-manager-tests

[belimawr]

Rebase onto main, force push

[cmacknz]

Needs to be backported to 8.10 this is failing there as well #36449 (comment)