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>

tests/e2e/e2e_test.go

@@ -37,7 +37,7 @@ func TestMain(m *testing.M) {
 	graceful.InitManager(managerCtx)
 	defer cancel()
 
-	tests.InitTest(false)
+	tests.InitTest()
 	testE2eWebRoutes = routers.NormalRoutes()
 
 	err := unittest.InitFixtures(