gce: fix test for non-existing instance in cloud

This commit fixes a GCE instance test for a scenario where instance does
not exist in the cloud. In the `gce_instances.go` we have a code that
handles such a scenario and strips off the error, i.e.

```
if providerID, err = cloudprovider.GetInstanceProviderID(ctx, g, types.NodeName(node.Name)); err != nil {
	if err == cloudprovider.InstanceNotFound {
		return false, nil
	}
	return false, err
}
```

but nevertheless the test was expecing a non-empty error string. This
issue got exposed when implementation of `GetInstanceProviderID` in the
`cloud-provider/cloud.go` changed to return `InstanceNotFound` as-is and
not as a custom string.

Signed-off-by: Mat Kowalski <mko@redhat.com>

staging/src/k8s.io/legacy-cloud-providers/gce/gce_instances_test.go

@@ -21,7 +21,6 @@ package gce
 
 import (
 	"context"
-	"fmt"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -55,7 +54,7 @@ func TestInstanceExists(t *testing.T) {
 			name:        "node not exist",
 			nodeName:    "test-node-2",
 			exist:       false,
-			expectedErr: fmt.Errorf("failed to get instance ID from cloud provider: instance not found"),
+			expectedErr: nil,
 		},
 	}