fix(spanner): make Priority in QueryOptions effective and add some related tests.

[tomo241]

Priority in QueryOptions is not considered in some functions in spanner/transaction.go. Currently, the following default Priority setting doesn't work and the read query is still run as high priority. I'm also adding some tests to check the effectiveness of the Priority setting.

client, err := spanner.NewClientWithConfig(ctx, dbPath, spanner.ClientConfig{
	QueryOptions: spanner.QueryOptions{Priority: sppb.RequestOptions_PRIORITY_MEDIUM},
})
 :
iter := client.Single().Query(ctx, stmt)
err = iter.Do(func(r *Row) error { return nil })