Split migrations folder (#21549)

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

models/migrations/v1_18/v230_test.go

@@ -0,0 +1,48 @@
+// Copyright 2022 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_18 //nolint
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/models/migrations/base"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_AddConfidentialClientColumnToOAuth2ApplicationTable(t *testing.T) {
+	// premigration
+	type OAuth2Application struct {
+		ID int64
+	}
+
+	// Prepare and load the testing database
+	x, deferable := base.PrepareTestEnv(t, 0, new(OAuth2Application))
+	defer deferable()
+	if x == nil || t.Failed() {
+		return
+	}
+
+	if err := AddConfidentialClientColumnToOAuth2ApplicationTable(x); err != nil {
+		assert.NoError(t, err)
+		return
+	}
+
+	// postmigration
+	type ExpectedOAuth2Application struct {
+		ID                 int64
+		ConfidentialClient bool
+	}
+
+	got := []ExpectedOAuth2Application{}
+	if err := x.Table("o_auth2_application").Select("id, confidential_client").Find(&got); !assert.NoError(t, err) {
+		return
+	}
+
+	assert.NotEmpty(t, got)
+	for _, e := range got {
+		assert.True(t, e.ConfidentialClient)
+	}
+}