Refactor database connection (#37496)

Clean up legacy copied&pasted code, introduce the unique "database
connection" function. Move migration testing helper function
PrepareTestEnv to a separate package.

By the way, remove "shadow connection secrets" tricks: showing
connection string on UI is useless

---------

Co-authored-by: Nicolas <bircni@icloud.com>

models/migrations/v1_18/v230_test.go

@@ -6,7 +6,7 @@ package v1_18
 import (
 	"testing"
 
-	"code.gitea.io/gitea/models/migrations/base"
+	"code.gitea.io/gitea/models/migrations/migrationtest"
 
 	"github.com/stretchr/testify/assert"
 )
@@ -18,7 +18,7 @@ func Test_AddConfidentialClientColumnToOAuth2ApplicationTable(t *testing.T) {
 	}
 
 	// Prepare and load the testing database
-	x, deferable := base.PrepareTestEnv(t, 0, new(oauth2Application))
+	x, deferable := migrationtest.PrepareTestEnv(t, 0, new(oauth2Application))
 	defer deferable()
 	if x == nil || t.Failed() {
 		return