Fix unrealistic FakeSampleStream.isReady

The FakeSampleStream is currently always ready even if it doesn't
have any samples to read. Fix that by checking for the conditions
under which read() will be successful.

PiperOrigin-RevId: 301371031

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

@@ -164,7 +164,7 @@ public final class FakeSampleStream implements SampleStream {
 
   @Override
   public boolean isReady() {
-    return true;
+    return !readFormat || readEOSBuffer || !fakeSampleStreamItems.isEmpty();
   }
 
   @Override