fix: prevent illegal negative timeout values into thread sleep() method while retrying exceptions in unit tests.

* For details on issue see - googleapis#2206

Author: arpan14

google-cloud-spanner/src/test/java/com/google/cloud/spanner/it/ITClosedSessionTest.java

@@ -251,7 +251,10 @@ public void testTransactionManager() throws InterruptedException {
             break;
           }
         } catch (AbortedException e) {
-          Thread.sleep(e.getRetryDelayInMillis());
+          long retryDelayInMillis = e.getRetryDelayInMillis();
+          if(retryDelayInMillis > 0) {
+            Thread.sleep(retryDelayInMillis);
+          }
           txn = manager.resetForRetry();
         }
       }