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/crash-fcntl-invalid-cmd.cpp

@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) 2020, Ben Wiederhake <BenWiederhake.GitHub@gmx.de>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <assert.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+int main(int, char**)
+{
+    int rc = fcntl(0, -42);
+    if (rc != -1) {
+        printf("FAIL: rc was %d, instead of -1\n", rc);
+        return 1;
+    } else if (errno != EINVAL) {
+        printf("FAIL: errno was %d, instead of EINVAL=%d\n", errno, EINVAL);
+        return 1;
+    } else {
+        printf("PASS\n");
+    }
+    return 0;
+}