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>

modules/gitrepo/main_test.go

@@ -13,12 +13,13 @@ import (
 
 func TestMain(m *testing.M) {
 	// resolve repository path relative to the test directory
-	testRootDir := setting.SetupGiteaTestEnv()
+	setting.SetupGiteaTestEnv()
+	giteaRoot := setting.GetGiteaTestSourceRoot()
 	repoPath = func(repo Repository) string {
 		if filepath.IsAbs(repo.RelativePath()) {
 			return repo.RelativePath() // for testing purpose only
 		}
-		return filepath.Join(testRootDir, "modules/git/tests/repos", repo.RelativePath())
+		return filepath.Join(giteaRoot, "modules/git/tests/repos", repo.RelativePath())
 	}
 	git.RunGitTests(m)
 }