Flaky test in TestS3WatcherTestSuite/TestStart due to race condition

[funkyshu]

Describe the bug
The TestS3WatcherTestSuite/TestStart test in contrib/vfsevents intermittently fails with an unexpected mock method call error. The test passes on retry, indicating a race condition in the test setup.

To Reproduce

1. Run the vfsevents test suite in CI: go test -v ./contrib/vfsevents/...
2. The TestS3WatcherTestSuite/TestStart test occasionally fails
3. Re-running the same test succeeds

Expected behavior
The test should pass consistently without flakiness.

Error Output

--- FAIL: TestS3WatcherTestSuite (0.26s)
    --- FAIL: TestS3WatcherTestSuite/TestStart (0.00s)
        mock.go:361: 
            assert: mock: I don't know what to return because the method call was unexpected.
            	Either do Mock.On("ReceiveMessage").Return(...) first, or remove the ReceiveMessage() call.
            	This method was unexpected:
            		ReceiveMessage(*context.cancelCtx,*sqs.ReceiveMessageInput)
            		0: &context.cancelCtx{...err:atomic.Value{v:(*errors.errorString)...}}
            		1: &sqs.ReceiveMessageInput{QueueUrl:(*string)(0x14000909c70), MaxNumberOfMessages:10, ...}

Additional context
The error shows ReceiveMessage was called with a canceled context, suggesting a race between:

• The watcher goroutine making a ReceiveMessage call after Start() is invoked
• The test canceling/stopping the watcher before mock expectations are fully exercised

Potential fixes:

• Add mock expectation for ReceiveMessage that handles the post-cancellation call
• Ensure context cancellation is checked before calling SQS methods
• Use mock.Anything for context matching to handle different context states
• Add synchronization between test setup and watcher goroutine startup