Adding code to ignore warning from error log.

mysql-test/lib/mtr_report.pl:
  Filtering out benign warning that can be generated during server shutdown.

mysql-test/lib/mtr_report.pl

@@ -279,6 +279,12 @@ sub mtr_report_stats ($) {
 		/Failed to open log/ or
 		/Failed to open the existing master info file/ or
 		/Forcing shutdown of [0-9]* plugins/ or
+
+		# Due to timing issues, it might be that this warning
+		# is printed when the server shuts down and the
+		# computer is loaded.
+		/Forcing close of thread \d+  user: '.*?'/ or
+
 		/Got error [0-9]* when reading table/ or
 		/Incorrect definition of table/ or
 		/Incorrect information in file/ or