[release-16.0] Fix bug in vtexplain around JOINs (#12376)

go/vt/vtexplain/testdata/multi-output/unsharded-output.txt

@@ -45,3 +45,8 @@ select ID from t1
 1 ks_unsharded/-: select ID from t1 limit 10001
 
 ----------------------------------------------------------------------
+select t1.id, t2.c2 from t1 join t2 on t1.id = t2.t1_id where t2.c2 in (1)
+
+1 ks_unsharded/-: select t1.id, t2.c2 from t1 join t2 on t1.id = t2.t1_id where t2.c2 in (1) limit 10001
+
+----------------------------------------------------------------------

go/vt/vtexplain/testdata/test-schema.sql

@@ -4,6 +4,12 @@ create table t1 (
 	floatval float not null default 0,
 	primary key (id)
 );
+create table t2 (
+	id bigint(20) unsigned not null,
+	t1_id bigint(20) unsigned not null default 0,
+	c2 bigint(20) null,
+	primary key (id)
+);
 
 create table user (
 	id bigint,

go/vt/vtexplain/testdata/test-vschema.json

@@ -3,6 +3,7 @@
 		"sharded": false,
 		"tables": {
 			"t1": {},
+			"t2": {},
 			"table_not_in_schema": {}
 		}
 	},

go/vt/vtexplain/testdata/unsharded-queries.sql

@@ -6,3 +6,4 @@ update t1 set floatval = 9.99;
 delete from t1 where id = 100;
 insert into t1 (id,intval,floatval) values (1,2,3.14) on duplicate key update intval=3, floatval=3.14;
 select ID from t1;
+select t1.id, t2.c2 from t1 join t2 on t1.id = t2.t1_id where t2.c2 in (1);