Add FakeSampleStreamItem to an existing FakeSampleStream.

Allows items to be added to the queue once the sample stream has already been created.
Means tests can simulate data not all being available at the start.

PiperOrigin-RevId: 290613392

testutils/src/main/java/com/google/android/exoplayer2/testutil/FakeSampleStream.java

@@ -133,6 +133,15 @@ public final class FakeSampleStream implements SampleStream {
     this.timeUs = timeUs;
   }
 
+  /**
+   * Adds an item to the end of the queue of {@link FakeSampleStreamItem items}.
+   *
+   * @param item The item to add.
+   */
+  public void addFakeSampleStreamItem(FakeSampleStreamItem item) {
+    this.fakeSampleStreamItems.add(item);
+  }
+
   @Override
   public boolean isReady() {
     return true;