AWS: Add a simple test for "shared" tag

For some reason this wasn't done in 0b5ae5391e.

pkg/cloudprovider/providers/aws/tags_test.go

@@ -57,6 +57,12 @@ func TestFindClusterID(t *testing.T) {
 			},
 			ExpectedNew: "a",
 		},
+		{
+			Tags: map[string]string{
+				TagNameKubernetesClusterPrefix + "a": "shared",
+			},
+			ExpectedNew: "a",
+		},
 		{
 			Tags: map[string]string{
 				TagNameKubernetesClusterPrefix + "a": "",