Fix recovering from invalid output configuration under Elastic-Agent

[belimawr]

What does this PR do?

This PR fixes two problems in the ManagerV2:

1. If the first output configuration was invalid, it would never be
   "saved", so if a new output configuration was received, the ManagerV2
   would try to reload it even with stopOnOutputReload enabled.

2. When stopOnOutputReload was set the output reload was skipped,
   but the ManagerV2 would still proceed with reloading inputs. This
   created a race condition where some inputs/runners would receive their
   stop signal before even start, effectively locking the shutdown
   process.

It also refactors some of the code used to mock the Elastic-Agent and
aggregates some shared code into a single package.

Why is it important?

• Fixes: Agents remain unhealthy on switching from invalid output to valid output. elastic-agent#2554

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.

## Author's Checklist

How to test this PR locally

Run the Go integration tests. Optionally you can also run the single test added by this PR:

cd x-pack/filebeat
mage buildSystemTestBinary && go test -v -count=1 -tags=integration -run=TestRecover ./tests/integration

Related issues

• Closes Agents remain unhealthy on switching from invalid output to valid output. elastic-agent#2554

## Use cases
## Screenshots
## Logs

[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]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[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-agent-unhealthy-on-broken-output upstream/fix-agent-unhealthy-on-broken-output
git merge upstream/main
git push upstream fix-agent-unhealthy-on-broken-output

[elasticmachine]

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Build stats

• Start Time: 2023-07-07T09:20:44.710+0000

• Duration: 93 min 25 sec

Test stats 🧪

Test	Results
Failed	0
Passed	27494
Skipped	2023
Total	29517

💚 Flaky test report

Tests succeeded.

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[cmacknz]

Tested locally, individually reverting the two fixes here (exiting reload when a restart is detected, unconditionally saving the output configuration) both cause the new test to fail.