[envtest]Stabilize racy testcase

The 8ff9207 introduced a racy test case
as it expected that the NovaCell status is up to date after the Cell's
Ready condition is True during NovaCompute delete. It turns out that it
is not true. So this patch wraps the test asserts to an Eventually
block to stabilize them.

The original issue could be reproduced locally with:
```
diff --git a/controllers/novacell_controller.go b/controllers/novacell_controller.go
index 4080438d..5f0e06bb 100644
--- a/controllers/novacell_controller.go
+++ b/controllers/novacell_controller.go
@@ -191,6 +191,7 @@ func (r *NovaCellReconciler) Reconcile(ctx context.Context, req ctrl.Request) (r
                        if err != nil {
                                return ctrl.Result{}, err
                        }
+                       time.Sleep(1 * time.Second)
                        delete(instance.Status.NovaComputesStatus, computeName)
                }
```

test/functional/novacell_controller_test.go

@@ -457,8 +457,10 @@ var _ = Describe("NovaCell controller", func() {
 				condition.ReadyCondition,
 				corev1.ConditionTrue,
 			)
-			novaCell = GetNovaCell(cell1.CellCRName)
-			Expect(novaCell.Status.NovaComputesStatus).To(BeEmpty())
+			Eventually(func(g Gomega) {
+				novaCell = GetNovaCell(cell1.CellCRName)
+				g.Expect(novaCell.Status.NovaComputesStatus).To(BeEmpty())
+			}, timeout, interval).Should(Succeed())
 
 		})
 	})