Split migrations folder (#21549)

There are too many files in `models/migrations` folder so that I split
them into sub folders.

models/migrations/v1_15/v182_test.go

@@ -0,0 +1,61 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package v1_15 //nolint
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/models/migrations/base"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_AddIssueResourceIndexTable(t *testing.T) {
+	// Create the models used in the migration
+	type Issue struct {
+		ID     int64 `xorm:"pk autoincr"`
+		RepoID int64 `xorm:"UNIQUE(s)"`
+		Index  int64 `xorm:"UNIQUE(s)"`
+	}
+
+	// Prepare and load the testing database
+	x, deferable := base.PrepareTestEnv(t, 0, new(Issue))
+	if x == nil || t.Failed() {
+		defer deferable()
+		return
+	}
+	defer deferable()
+
+	// Run the migration
+	if err := AddIssueResourceIndexTable(x); err != nil {
+		assert.NoError(t, err)
+		return
+	}
+
+	type ResourceIndex struct {
+		GroupID  int64 `xorm:"pk"`
+		MaxIndex int64 `xorm:"index"`
+	}
+
+	start := 0
+	const batchSize = 1000
+	for {
+		indexes := make([]ResourceIndex, 0, batchSize)
+		err := x.Table("issue_index").Limit(batchSize, start).Find(&indexes)
+		assert.NoError(t, err)
+
+		for _, idx := range indexes {
+			var maxIndex int
+			has, err := x.SQL("SELECT max(`index`) FROM issue WHERE repo_id = ?", idx.GroupID).Get(&maxIndex)
+			assert.NoError(t, err)
+			assert.True(t, has)
+			assert.EqualValues(t, maxIndex, idx.MaxIndex)
+		}
+		if len(indexes) < batchSize {
+			break
+		}
+		start += len(indexes)
+	}
+}