Tweak the assertions in MediaSourceTestRunner to give clearer failures

PiperOrigin-RevId: 300519989

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

@@ -215,7 +215,7 @@ public class MediaSourceTestRunner {
    * runner was created if neither method has been called).
    */
   public void assertNoTimelineChange() {
-    assertThat(timelines.isEmpty()).isTrue();
+    assertThat(timelines).isEmpty();
   }
 
   /**
@@ -304,8 +304,10 @@ public class MediaSourceTestRunner {
     List<Integer> expectedWindowIndices = new ArrayList<>(Arrays.asList(windowIndices));
     for (Pair<Integer, MediaPeriodId> windowIndexAndMediaPeriodId : completedLoads) {
       if (windowIndexAndMediaPeriodId.second == null) {
-        boolean loadExpected = expectedWindowIndices.remove(windowIndexAndMediaPeriodId.first);
-        assertThat(loadExpected).isTrue();
+        assertWithMessage("Missing expected load")
+            .that(expectedWindowIndices)
+            .contains(windowIndexAndMediaPeriodId.first);
+        expectedWindowIndices.remove(windowIndexAndMediaPeriodId.first);
       }
     }
     assertWithMessage("Not all expected media source loads have been completed.")