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_17/main_test.go

@@ -6,9 +6,9 @@ package v1_17
 import (
 	"testing"
 
-	"code.gitea.io/gitea/models/migrations/base"
+	"code.gitea.io/gitea/models/migrations/migrationtest"
 )
 
 func TestMain(m *testing.M) {
-	base.MainTest(m)
+	migrationtest.MainTest(m)
 }

models/migrations/v1_17/v221_test.go

@@ -7,7 +7,7 @@ import (
 	"encoding/base32"
 	"testing"
 
-	"code.gitea.io/gitea/models/migrations/base"
+	"code.gitea.io/gitea/models/migrations/migrationtest"
 
 	"github.com/stretchr/testify/assert"
 )
@@ -38,7 +38,7 @@ func Test_StoreWebauthnCredentialIDAsBytes(t *testing.T) {
 	}
 
 	// Prepare and load the testing database
-	x, deferable := base.PrepareTestEnv(t, 0, new(WebauthnCredential), new(ExpectedWebauthnCredential))
+	x, deferable := migrationtest.PrepareTestEnv(t, 0, new(WebauthnCredential), new(ExpectedWebauthnCredential))
 	defer deferable()
 	if x == nil || t.Failed() {
 		return