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/unveil-symlinks.cpp

@@ -0,0 +1,76 @@
+/*
+ * Copyright (c) 2020, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <fcntl.h>
+#include <stdio.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+int main()
+{
+    rmdir("/tmp/foo/1");
+    rmdir("/tmp/foo");
+    unlink("/tmp/bar");
+
+    if (mkdir("/tmp/foo", 0755) < 0) {
+        perror("mkdir");
+        return 1;
+    }
+
+    if (mkdir("/tmp/foo/1", 0755) < 0) {
+        perror("mkdir");
+        return 1;
+    }
+
+    if (symlink("/tmp/foo", "/tmp/bar")) {
+        perror("symlink");
+        return 1;
+    }
+
+    if (unveil("/tmp/foo", "r") < 0) {
+        perror("unveil");
+        return 1;
+    }
+
+    if (unveil(nullptr, nullptr) < 0) {
+        perror("unveil");
+        return 1;
+    }
+
+    int fd = open("/tmp/foo/1", O_RDONLY);
+    if (fd < 0) {
+        perror("open");
+        return 1;
+    }
+    close(fd);
+
+    fd = open("/tmp/bar/1", O_RDONLY);
+    if (fd >= 0) {
+        fprintf(stderr, "FAIL, symlink was not unveiled\n");
+        return 1;
+    }
+
+    if (chdir("/tmp")) {
+        perror("chdir");
+        return 1;
+    }
+
+    fd = open("./foo/1", O_RDONLY);
+    if (fd < 0) {
+        perror("open");
+        return 1;
+    }
+    close(fd);
+
+    fd = open("./bar/1", O_RDONLY);
+    if (fd >= 0) {
+        fprintf(stderr, "FAIL, symlink was not unveiled\n");
+        return 1;
+    }
+
+    printf("PASS\n");
+    return 0;
+}