Tests: Establish root Tests directory, move Userland/Tests there

With the goal of centralizing all tests in the system, this is a
first step to establish a Tests sub-tree. It will contain all of
the unit tests and test harnesses for the various components in the
system.

Tests/LibC/stack-smash.cpp

@@ -0,0 +1,34 @@
+/*
+ * Copyright (c) 2021, Brian Gianforcaro <bgianf@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <cstdio>
+
+// Note: Needs to be 'noline' so stack canary isn't optimized out.
+static void __attribute__((noinline)) smasher(char* string)
+{
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Warray-bounds"
+    for (int i = 0; i < 256; i++) {
+        string[i] = 'A';
+    }
+#pragma GCC diagnostic pop
+}
+
+// Note: Needs to be 'noline' so stack canary isn't optimized out.
+static void __attribute__((noinline)) stack_to_smash()
+{
+    char string[8] = {};
+    smasher(string);
+}
+
+int main()
+{
+    puts("[+] Starting the stack smash...");
+    stack_to_smash();
+    puts("[+] Stack smash wasn't detected!");
+
+    return 0;
+}