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/testlogger/testlogger.go

@@ -173,7 +173,7 @@ func Init() {
 	log.RegisterEventWriter("test", newTestLoggerWriter)
 }
 
-func Fatalf(format string, args ...any) {
-	Printf(format+"\n", args...)
-	os.Exit(1)
+func Panicf(format string, args ...any) {
+	// don't call os.Exit, otherwise the "defer" functions won't be executed
+	panic(fmt.Sprintf(format, args...))
 }