Refactor to allow testing side effects

By externalizing the source of i/o and randomness for shuffling, we can
inject non-interactive and deterministic behavior during unit tests.

10_Blackjack/java/test/DeckTest.java

@@ -7,7 +7,8 @@ public class DeckTest {
     @Test
     void testInit() {
         // When
-        Deck deck = new Deck();
+        Deck deck = new Deck((cards) -> cards);
+        deck.reshuffle();
 
         // Then
         long nCards = deck.size();