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_27/main_test.go

@@ -6,9 +6,9 @@ package v1_27
 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_27/v331_test.go

@@ -8,7 +8,7 @@ import (
 	"slices"
 	"testing"
 
-	"code.gitea.io/gitea/models/migrations/base"
+	"code.gitea.io/gitea/models/migrations/migrationtest"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -49,7 +49,7 @@ func (actionArtifactBeforeV331) TableName() string {
 }
 
 func Test_AddActionRunAttemptModel(t *testing.T) {
-	x, deferable := base.PrepareTestEnv(t, 0,
+	x, deferable := migrationtest.PrepareTestEnv(t, 0,
 		new(actionRunBeforeV331),
 		new(actionRunJobBeforeV331),
 		new(actionArtifactBeforeV331),
@@ -69,7 +69,7 @@ func Test_AddActionRunAttemptModel(t *testing.T) {
 
 	require.NoError(t, AddActionRunAttemptModel(x))
 
-	tableMap := base.LoadTableSchemasMap(t, x)
+	tableMap := migrationtest.LoadTableSchemasMap(t, x)
 
 	attemptTable := tableMap["action_run_attempt"]
 	require.NotNil(t, attemptTable)