Move FakeInput tests into their own test suite

[faec]

Tests using the FakeInput and FakeShipper components need to run Agent test code in a temporary test directory. To do this, they need to modify the global flags unversionedHome and topPath. However, they can't safely restore these flags at the end of the test, because it is detected as a data race with access of these flags during the test execution (logically this is not a real race but the race detector doesn't know that).

This PR resolves that by moving these tests into their own test suite, which sets the correct temporary paths at the beginning of each test instead of through a manual helper function call. This way the globals don't need to be explicitly cleaned up after the tests, and there's less risk of omitting the temporary directory setup in a test where it's needed.

The diff is less than it looks like, since (other than moving to a different file) most tests only have changes in their first couple lines.

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

Related issues

• Fixes Fix data race in tests that modify global agent paths in component runtime tests #3683

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[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.

[elastic-sonarqube]

Quality Gate passed

Kudos, no new issues were introduced!

0 New issues
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarQube

[pierrehilbert]

Unrelated failure #4076

[pierrehilbert]

@faec do we need another round of review or should I force merge this one as the failure is unrelated to this change?