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/bind-local-socket-to-symlink.cpp

@@ -0,0 +1,40 @@
+/*
+ * Copyright (c) 2018-2020, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Assertions.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/socket.h>
+#include <unistd.h>
+
+int main(int, char**)
+{
+    constexpr const char* path = "/tmp/foo";
+    int rc = symlink("bar", path);
+    if (rc < 0) {
+        perror("symlink");
+        return 1;
+    }
+
+    int fd = socket(AF_UNIX, SOCK_STREAM, 0);
+    if (fd < 0) {
+        perror("socket");
+        return 1;
+    }
+
+    struct sockaddr_un addr;
+    memset(&addr, 0, sizeof(addr));
+    addr.sun_family = AF_UNIX;
+    VERIFY(strlcpy(addr.sun_path, path, sizeof(addr.sun_path)) < sizeof(addr.sun_path));
+
+    rc = bind(fd, (struct sockaddr*)(&addr), sizeof(addr));
+    if (rc < 0 && errno == EADDRINUSE) {
+        printf("PASS\n");
+        return 0;
+    }
+
+    return 1;
+}