MDEV-7558 analyze_stmt_slow_query_log fails sporadically in buildbot

The reason was that the test was reusing the same log file without deleting it between tests.
Fixed by creating a new log file as part of the test

mysql-test/r/analyze_stmt_slow_query_log.result

 drop table if exists t1;
+SET @@global.slow_query_log = OFF;
+FLUSH SLOW LOGS;
+SET @@global.slow_query_log = ON;
 create table t1 (a int);
 INSERT INTO t1 VALUES (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
 select * from t1 where a<3;

mysql-test/t/analyze_stmt_slow_query_log.test

@@ -2,6 +2,21 @@
 drop table if exists t1;
 --enable_warnings
 
+#
+# Remove old log file
+#
+let SLOW_LOG_FILE= `select @@slow_query_log_file`;
+
+SET @@global.slow_query_log = OFF;
+
+perl;
+  my $slow_log_file= $ENV{'SLOW_LOG_FILE'} or die "SLOW_LOG_FILE not set";
+  unlink($slow_log_file);
+EOF
+
+FLUSH SLOW LOGS;
+SET @@global.slow_query_log = ON;
+
 create table t1 (a int);
 INSERT INTO t1 VALUES (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
 select * from t1 where a<3;