fix TX leak in testCommitReadTxException()

Author: greenrobot

tests/objectbox-java-test/src/main/java/io/objectbox/TransactionTest.java

@@ -176,7 +176,11 @@ public void testCommitAfterAbortException() {
     @Test(expected = IllegalStateException.class)
     public void testCommitReadTxException() {
         Transaction tx = store.beginReadTx();
-        tx.commit();
+        try {
+            tx.commit();
+        } finally {
+            tx.abort();
+        }
     }
 
     /*