Fix `ExoPlayerTest` to use `C.TIME_UNSET` instead of `C.POSITION_UNSET`

This inconsistency was exposed by an upcoming change to deprecate
`POSITION_UNSET` in favour of `INDEX_UNSET` because position is an
ambiguous term between 'byte offset' and 'media position', as shown
here.

PiperOrigin-RevId: 492470241

library/core/src/test/java/com/google/android/exoplayer2/ExoPlayerTest.java

@@ -2514,7 +2514,7 @@ public final class ExoPlayerTest {
         .build()
         .start()
         .blockUntilEnded(TIMEOUT_MS);
-    assertThat(target.positionMs).isEqualTo(C.POSITION_UNSET);
+    assertThat(target.positionMs).isEqualTo(C.TIME_UNSET);
   }
 
   @Test
@@ -2536,7 +2536,7 @@ public final class ExoPlayerTest {
         .build()
         .start()
         .blockUntilEnded(TIMEOUT_MS);
-    assertThat(target.positionMs).isEqualTo(C.POSITION_UNSET);
+    assertThat(target.positionMs).isEqualTo(C.TIME_UNSET);
   }
 
   @Test
@@ -12282,7 +12282,7 @@ public final class ExoPlayerTest {
 
     public PositionGrabbingMessageTarget() {
       mediaItemIndex = C.INDEX_UNSET;
-      positionMs = C.POSITION_UNSET;
+      positionMs = C.TIME_UNSET;
     }
 
     @Override