Fix Gradle-only failure in MkvPlaybackTest with muxed subrip subs

This ensures that the subtitles are fully loaded and parsed before
playback starts.

This failure was flagged in #2628 (comment)

PiperOrigin-RevId: 787143300

Author: icbaker

libraries/exoplayer/src/test/java/androidx/media3/exoplayer/e2etest/MkvPlaybackTest.java

@@ -15,6 +15,7 @@
  */
 package androidx.media3.exoplayer.e2etest;
 
+import static androidx.media3.test.utils.robolectric.TestPlayerRunHelper.advance;
 import static org.robolectric.annotation.GraphicsMode.Mode.NATIVE;
 
 import android.content.Context;
@@ -29,7 +30,6 @@
 import androidx.media3.test.utils.FakeClock;
 import androidx.media3.test.utils.robolectric.PlaybackOutput;
 import androidx.media3.test.utils.robolectric.ShadowMediaCodecConfig;
-import androidx.media3.test.utils.robolectric.TestPlayerRunHelper;
 import androidx.test.core.app.ApplicationProvider;
 import com.google.common.collect.ImmutableList;
 import org.junit.Rule;
@@ -83,8 +83,9 @@ public void test() throws Exception {
 
     player.setMediaItem(MediaItem.fromUri("asset:///media/mkv/" + inputFile));
     player.prepare();
+    advance(player).untilFullyBuffered();
     player.play();
-    TestPlayerRunHelper.runUntilPlaybackState(player, Player.STATE_ENDED);
+    advance(player).untilState(Player.STATE_ENDED);
     player.release();
     surface.release();