MDEV-14380 Reduce possibility to timing-induced error in test

Also, shorten the test so it usually runs 2 seconds, not 3.

mysql-test/main/set_statement_notembedded.result

 SELECT @@MAX_STATEMENT_TIME;
 @@MAX_STATEMENT_TIME
 0.000000
-SET STATEMENT MAX_STATEMENT_TIME=2 FOR SELECT SLEEP(1);
+SET STATEMENT MAX_STATEMENT_TIME=3 FOR SELECT SLEEP(1);
 SLEEP(1)
 0
 SHOW STATUS LIKE "max_statement_time_exceeded";
 Variable_name	Value
 Max_statement_time_exceeded	0
-SET STATEMENT MAX_STATEMENT_TIME=2 FOR SELECT SLEEP(3);
+SET STATEMENT MAX_STATEMENT_TIME=1 FOR SELECT SLEEP(3);
 SLEEP(3)
 1
 SHOW STATUS LIKE "max_statement_time_exceeded";

mysql-test/main/set_statement_notembedded.test

@@ -4,9 +4,9 @@
 # statement timeout
 #
 SELECT @@MAX_STATEMENT_TIME;
-SET STATEMENT MAX_STATEMENT_TIME=2 FOR SELECT SLEEP(1);
+SET STATEMENT MAX_STATEMENT_TIME=3 FOR SELECT SLEEP(1);
 SHOW STATUS LIKE "max_statement_time_exceeded";
-SET STATEMENT MAX_STATEMENT_TIME=2 FOR SELECT SLEEP(3);
+SET STATEMENT MAX_STATEMENT_TIME=1 FOR SELECT SLEEP(3);
 SHOW STATUS LIKE "max_statement_time_exceeded";
 SELECT @@MAX_STATEMENT_TIME;