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/timeutil/since_test.go

@@ -32,11 +32,7 @@ func TestMain(m *testing.M) {
 	// setup
 	translation.InitLocales(context.Background())
 	BaseDate = time.Date(2000, time.January, 1, 0, 0, 0, 0, time.UTC)
-
-	// run the tests
-	retVal := m.Run()
-
-	os.Exit(retVal)
+	os.Exit(m.Run())
 }
 
 func TestTimeSincePro(t *testing.T) {