MDEV-8518 rpl.sec_behind_master-5114 fails sporadically in buildbot

- fix the test to avoid false-negatives before MDEV-5114 patch;
- fix the race condition which made the test fail on slow builders

mysql-test/suite/rpl/r/sec_behind_master-5114.result

@@ -3,7 +3,8 @@ include/master-slave.inc
 call mtr.add_suppression("Unsafe statement written to the binary log");
 CREATE TABLE t1 (a int);
 INSERT INTO t1 VALUES(SLEEP(2));
-Seconds_Behind_Master: 1
+Seconds_Behind_Master_is_less_than_100
+1
 Warnings:
 Note	1592	Unsafe statement written to the binary log using statement format since BINLOG_FORMAT = STATEMENT. Statement is unsafe because it uses a system function that may return a different value on the slave.
 drop table t1;

mysql-test/suite/rpl/t/sec_behind_master-5114.test

@@ -3,24 +3,60 @@ source include/have_binlog_format_statement.inc;
 
 call mtr.add_suppression("Unsafe statement written to the binary log");
 
+
+# Make sure that the start time of the first event is certainly different
+# from the next event
+--let $timestamp= `SELECT @@timestamp`
+--disable_query_log
+eval SET TIMESTAMP= $timestamp-100;
+--enable_query_log
 CREATE TABLE t1 (a int);
+
+# Make sure that the slave is done with the first event, and all checks
+# that we'll perform later will be really against the second event
+sync_slave_with_master;
+
+connection master;
+
+# Restore the timestamp now. It doesn't matter that it's not precise,
+# it just needs to be very different from the earlier event
+--disable_query_log
+eval SET TIMESTAMP= $timestamp;
+--enable_query_log
+
 send INSERT INTO t1 VALUES(SLEEP(2));
 
 connection slave;
-let $run = 10;
+
+# When the slave starts executing the event, Seconds_Behind_Master
+# should start growing steadilly. The bugfix ensures that they are
+# calculated based on the start time of the current event, rather
+# than the start time of the previous event. To check it, we only need
+# the first non-zero value
+
+let $run = 20;
 while ($run)
 {
   dec $run;
   let $sbm=query_get_value(SHOW SLAVE STATUS, Seconds_Behind_Master, 1);
   # for debugging uncomment echo and remove the if()
-  #echo Seconds_Behind_Master: $sbm;
+#  echo Seconds_Behind_Master: $sbm;
   if ($sbm)
   {
     let $run = 0;
   }
   sleep 0.5;
 }
-echo Seconds_Behind_Master: $sbm;
+
+# Normally the first non-zero value should be 1. However, due to race
+# conditions on slow servers, sometimes the check might miss the value 1,
+# and only catch a higher one. It does not matter, we just need to make
+# sure it didn't start with 100+, as it would have with bug MDEV-5114
+
+--disable_query_log
+eval SELECT $sbm > 0 and $sbm < 99 AS Seconds_Behind_Master_is_less_than_100;
+--enable_query_log
+
 connection master;
 reap;
 drop table t1;