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/path-resolution-race.cpp

@@ -0,0 +1,22 @@
+/*
+ * Copyright (c) 2018-2020, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <sys/stat.h>
+#include <unistd.h>
+
+int main()
+{
+    if (!fork()) {
+        for (;;) {
+            mkdir("/tmp/x", 0666);
+            rmdir("/tmp/x");
+        }
+    }
+    for (;;) {
+        chdir("/tmp/x");
+    }
+    return 0;
+}