Fix Translog#testWithRandomException (#51712)

Before #51417, trimUnreferencedReaders in this test was a noop as the 
default translog retention policy was 512MB and 12h, which was long 
enough to keep all translog files. Two problems in this test:

- We do not consider any failure while trimming unreferenced readers as 
  a tragedy.

- We should clear the synced list before calling 
  trimUnreferencedReaders.

Relates #51505
Closes #51694

Author: dnhatn

server/src/test/java/org/elasticsearch/index/translog/TranslogTests.java

@@ -2657,9 +2657,9 @@ public void testWithRandomException() throws IOException {
                             committing = true;
                             failableTLog.getDeletionPolicy().setTranslogGenerationOfLastCommit(failableTLog.currentFileGeneration());
                             failableTLog.getDeletionPolicy().setMinTranslogGenerationForRecovery(failableTLog.currentFileGeneration());
+                            syncedDocs.clear();
                             failableTLog.trimUnreferencedReaders();
                             committing = false;
-                            syncedDocs.clear();
                         }
                     }
                     // we survived all the randomness!!!
@@ -2675,7 +2675,7 @@ public void testWithRandomException() throws IOException {
                     assertEquals(failableTLog.getTragicException(), ex);
                 } catch (RuntimeException ex) {
                     assertEquals(ex.getMessage(), "simulated");
-                    assertEquals(failableTLog.getTragicException(), ex);
+                    assertNull("Don't consider failures while trimming unreferenced readers as tragedy", failableTLog.getTragicException());
                 } finally {
                     Checkpoint checkpoint = Translog.readCheckpoint(config.getTranslogPath());
                     if (checkpoint.numOps == unsynced.size() + syncedDocs.size()) {