Flaky unit test for the placement scheduling

[zhujian7]

--- FAIL: TestBind (0.02s)
    --- FAIL: TestBind/create_placementdecision_with_only_cluster_per_decision_group (0.00s)
        scheduling_controller_test.go:1429: expected clusters selected: cluster1, but got [{cluster3 }]
        scheduling_controller_test.go:1429: expected clusters selected: cluster2, but got [{cluster1 }]
        scheduling_controller_test.go:1429: expected clusters selected: cluster3, but got [{cluster2 }]
FAIL
        open-cluster-management.io/ocm/pkg/placement/controllers/scheduling     coverage: 75.5% of statements
FAIL    open-cluster-management.io/ocm/pkg/placement/controllers/scheduling     0.298s

see https://github.com/open-cluster-management-io/ocm/actions/runs/5654038071/job/15316306014?pr=231

cc @haoqing0110

[qiujian16]

I see this error sometimes also

/kind bug
/assign @haoqing0110

[openshift-ci]

@qiujian16: The label(s) kind/bug cannot be applied, because the repository doesn't have them.

In response to this:

I see this error sometimes also

/kind bug
/assign @haoqing0110

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[haoqing0110]

Will fix in #235