Fix edge case in DASH tests

During a refactor ("Replace find/get callbacks with SegmentIndex"),
this test changed from using a hard-coded segment position to looking
one up based on the live edge.  But using the exact live edge leads to
some amount of flakiness.

Back off the live edge enough to make sure you're not on the trailing
edge of the segment.

Fixes b/137286528

Change-Id: Ic4e869e8a3b1ceaf45dc14b04ea0e987bd690453

Author: joeyparrish

test/dash/dash_parser_live_unit.js

@@ -1029,11 +1029,18 @@ describe('DashParser Live', () => {
 
       const liveEdge =
           manifest.presentationTimeline.getSegmentAvailabilityEnd();
-      const idx = stream.segmentIndex.find(liveEdge);
 
-      // In https://github.com/google/shaka-player/issues/1204, this
-      // failed an assertion and returned endTime == 0.
+      // In https://github.com/google/shaka-player/issues/1204, a get on the
+      // final segment failed an assertion and returned endTime == 0.
+      // Find the last segment by looking just before the live edge.  Looking
+      // right on the live edge creates test flake, and the segments are 2
+      // seconds in duration.
+      const idx = stream.segmentIndex.find(liveEdge - 0.5);
+      expect(idx).not.toBe(null);
+
+      // This should not throw an assertion.
       const ref = stream.segmentIndex.get(idx);
+      // The segment's endTime should definitely not be 0.
       expect(ref.endTime).toBeGreaterThan(0);
     });
   });