MDEV-6388: ANALYZE $stmt output in the slow query log

Add testcase.

mysql-test/r/analyze_stmt_slow_query_log.result

+drop table if exists t1;
+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;
+a
+0
+1
+2
+drop table t1;
+# explain: id	select_type	table	type	possible_keys	key	key_len	ref	rows	r_rows	filtered	r_filtered	Extra
+# explain: 1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+# explain: id	select_type	table	type	possible_keys	key	key_len	ref	rows	r_rows	filtered	r_filtered	Extra
+# explain: 1	INSERT	t1	ALL	NULL	NULL	NULL	NULL	NULL	NULL	100.00	100.00	NULL
+# explain: id	select_type	table	type	possible_keys	key	key_len	ref	rows	r_rows	filtered	r_filtered	Extra
+# explain: 1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	10	10	100.00	30.00	Using where
+# explain: id	select_type	table	type	possible_keys	key	key_len	ref	rows	r_rows	filtered	r_filtered	Extra
+# explain: 1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used

mysql-test/t/analyze_stmt_slow_query_log-master.opt

+--slow-query-log --long-query-time=0.00001 --log-slow-verbosity=query_plan,explain

mysql-test/t/analyze_stmt_slow_query_log.test

+--disable_warnings
+drop table if exists t1;
+--enable_warnings
+
+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;
+drop table t1;
+let SLOW_LOG_FILE= `select @@slow_query_log_file`;
+
+# select @@slow_query_log_file;
+
+perl;
+
+  my $slow_log_file= $ENV{'SLOW_LOG_FILE'} or die "SLOW_LOG_FILE not set";
+  open(FILE, $slow_log_file) or die "Failed to open $slow_log_file";
+  while(<FILE>) {
+    if (/explain:/) {
+      print $_;
+    }
+  }
+  close(FILE);
+
+EOF