Add assertions to improve clarity of unit test

Signed-off-by: Andrew Ross <andrross@amazon.com>

server/src/test/java/org/opensearch/index/store/remote/utils/TransferManagerTests.java

@@ -20,6 +20,7 @@
 import org.apache.lucene.store.IndexInput;
 import org.apache.lucene.store.MMapDirectory;
 import org.apache.lucene.store.SimpleFSLockFactory;
+import org.hamcrest.MatcherAssert;
 import org.junit.After;
 import org.junit.Before;
 import org.opensearch.common.blobstore.BlobContainer;
@@ -30,6 +31,7 @@
 
 import com.carrotsearch.randomizedtesting.annotations.ThreadLeakFilters;
 
+import static org.hamcrest.Matchers.equalTo;
 import static org.mockito.Mockito.doAnswer;
 import static org.mockito.Mockito.mock;
 
@@ -60,29 +62,35 @@ public void tearDown() throws Exception {
     public void testSingleAccess() throws IOException {
         try (IndexInput i = fetchBlob()) {
             i.seek(7);
+            MatcherAssert.assertThat(i.readByte(), equalTo((byte) 7));
         }
     }
 
     public void testConcurrentAccess() throws Exception {
         // Kick off multiple threads that all concurrently request the same resource
         final ExecutorService testRunner = Executors.newFixedThreadPool(8);
-        final List<Future<IndexInput>> futures = new ArrayList<>();
-        for (int i = 0; i < 8; i++) {
-            futures.add(testRunner.submit(this::fetchBlob));
+        try {
+            final List<Future<IndexInput>> futures = new ArrayList<>();
+            for (int i = 0; i < 8; i++) {
+                futures.add(testRunner.submit(this::fetchBlob));
+            }
+            // Wait for all threads to complete
+            for (Future<IndexInput> future : futures) {
+                future.get(1, TimeUnit.SECONDS);
+            }
+            // Assert that all IndexInputs are independently positioned by seeking
+            // to the end and closing each one. If not independent, then this would
+            // result in EOFExceptions and/or NPEs.
+            for (Future<IndexInput> future : futures) {
+                try (IndexInput i = future.get()) {
+                    i.seek(7);
+                    MatcherAssert.assertThat(i.readByte(), equalTo((byte) 7));
+                }
+            }
+        } finally {
+            testRunner.shutdown();
+            assertTrue(testRunner.awaitTermination(1, TimeUnit.SECONDS));
         }
-        // Wait for all threads to complete
-        for (Future<IndexInput> future : futures) {
-            future.get(1, TimeUnit.SECONDS);
-        }
-        // Assert that all IndexInputs are independently positioned by seeking
-        // to the end and closing each one. If not independent, then this would
-        // result in EOFExceptions and/or NPEs.
-        for (Future<IndexInput> future : futures) {
-            future.get().seek(7);
-            future.get().close();
-        }
-        testRunner.shutdown();
-        assertTrue(testRunner.awaitTermination(1, TimeUnit.SECONDS));
     }
 
     private IndexInput fetchBlob() {