Clean up Makefile, tests and legacy code (#36638)

This simplifies the Makefile by removing the whole-file wrapping that
creates a tempdir introduced by
https://github.com/go-gitea/gitea/pull/11126. REPO_TEST_DIR is removed
as well.

Also clean up a lot of legacy code: unnecessary XSS test, incorrect test
env init, unused "_old_uid" hack, etc

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

models/unittest/fixtures_test.go

@@ -4,6 +4,7 @@
 package unittest_test
 
 import (
+	"os"
 	"path/filepath"
 	"testing"
 
@@ -58,9 +59,14 @@ func NewFixturesLoaderVendorGoTestfixtures(e *xorm.Engine, opts unittest.Fixture
 }
 */
 
+func TestMain(m *testing.M) {
+	setting.SetupGiteaTestEnv()
+	os.Exit(m.Run())
+}
+
 func prepareTestFixturesLoaders(t testing.TB) unittest.FixturesOptions {
 	_ = user_model.User{}
-	giteaRoot := setting.SetupGiteaTestEnv()
+	giteaRoot := setting.GetGiteaTestSourceRoot()
 	opts := unittest.FixturesOptions{Dir: filepath.Join(giteaRoot, "models", "fixtures"), Files: []string{
 		"user.yml",
 	}}