Refactor directory and file setup checks in tests

[tiffanny29631]

This commit refactors a more deterministic approach using channels to signal the completion of setup tasks. This change ensures that the tests proceed only after the necessary conditions are met or fail immediately upon setup errors, thus improving test stability.

[google-oss-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please ask for approval from tiffanny29631. For more information see the Kubernetes Code Review Process.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[sdowell]

What's the purpose of doing the setup in a separate goroutine if we immediately block for the goroutine to finish? My understanding is that the intent of this test was to write to the filesystem async while SourceCommitAndDirWithRetry is running to simulate the sidecar behavior

[nan-yu]

What's the intent of this commit, fixing the flakiness in TestSourceCommitAndDirWithRetry?

I took a look at the presubmit failures in this month. Besides a bunch of linter errors, there're 12 test failures for TestRoot_SourceReconcilerErrorsMetricValidation, and 1 test failure for TestBackoffRetryCount. It would be more helpful to stabilize the other two unit tests.