sql: Add regression test for #2251

Add regression test for the #2251 issue.
Problem was resolved earlier.

Closes #2251

test/sql/gh2251-multiple-update.result

+-- Regression test for #2251
+test_run = require('test_run').new()
+---
+...
+-- box.cfg()
+box.sql.execute("CREATE TABLE t1(a integer primary key, b UNIQUE, e);")
+---
+...
+box.sql.execute("INSERT INTO t1 VALUES(1,4,6);")
+---
+...
+box.sql.execute("INSERT INTO t1 VALUES(2,5,7);")
+---
+...
+box.sql.execute("UPDATE t1 SET e=e+1 WHERE b IN (SELECT b FROM t1);")
+---
+...
+box.sql.execute("SELECT e FROM t1")
+---
+- - [7]
+  - [8]
+...
+box.sql.execute("CREATE TABLE t2(a integer primary key, b UNIQUE, c, d, e,  UNIQUE(c,d));")
+---
+...
+box.sql.execute("INSERT INTO t2 VALUES(1,2,3,4,5);")
+---
+...
+box.sql.execute("INSERT INTO t2 VALUES(2,3,4,4,6);")
+---
+...
+box.sql.execute("UPDATE t2 SET e=e+1 WHERE b IN (SELECT b FROM t2);")
+---
+...
+box.sql.execute("SELECT e FROM t2")
+---
+- - [6]
+  - [7]
+...
+box.sql.execute("DROP TABLE t1")
+---
+...
+box.sql.execute("DROP TABLE t2")
+---
+...
+-- Debug
+-- require("console").start()

test/sql/gh2251-multiple-update.test.lua

+-- Regression test for #2251
+test_run = require('test_run').new()
+
+-- box.cfg()
+
+box.sql.execute("CREATE TABLE t1(a integer primary key, b UNIQUE, e);")
+box.sql.execute("INSERT INTO t1 VALUES(1,4,6);")
+box.sql.execute("INSERT INTO t1 VALUES(2,5,7);")
+
+box.sql.execute("UPDATE t1 SET e=e+1 WHERE b IN (SELECT b FROM t1);")
+
+box.sql.execute("SELECT e FROM t1")
+
+box.sql.execute("CREATE TABLE t2(a integer primary key, b UNIQUE, c, d, e,  UNIQUE(c,d));")
+box.sql.execute("INSERT INTO t2 VALUES(1,2,3,4,5);")
+box.sql.execute("INSERT INTO t2 VALUES(2,3,4,4,6);")
+
+box.sql.execute("UPDATE t2 SET e=e+1 WHERE b IN (SELECT b FROM t2);")
+
+box.sql.execute("SELECT e FROM t2")
+
+box.sql.execute("DROP TABLE t1")
+box.sql.execute("DROP TABLE t2")
+
+-- Debug
+-- require("console").start()