Stronger check in translog prepare and commit test

This commit strengthens an assertion in the translog prepare commit and
commit test.

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

@@ -2273,7 +2273,7 @@ public class TranslogTests extends ESTestCase {
                 for (long g = 0; g < committedGeneration; g++) {
                     assertFileDeleted(translog, g);
                 }
-                for (long g = committedGeneration; g < translog.currentFileGeneration(); g++) {
+                for (long g = committedGeneration; g <= translog.currentFileGeneration(); g++) {
                     assertFileIsPresent(translog, g);
                 }
             }