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/null-deref-close-during-select.cpp

@@ -0,0 +1,42 @@
+/*
+ * Copyright (c) 2018-2020, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <pthread.h>
+#include <stdio.h>
+#include <sys/select.h>
+#include <unistd.h>
+
+int pipefds[2];
+
+int main(int, char**)
+{
+    pipe(pipefds);
+
+    pthread_t tid;
+    pthread_create(
+        &tid, nullptr, [](void*) -> void* {
+            sleep(1);
+            printf("ST: close()\n");
+            close(pipefds[1]);
+            pthread_exit(nullptr);
+            return nullptr;
+        },
+        nullptr);
+
+    fd_set rfds;
+    FD_ZERO(&rfds);
+    FD_SET(pipefds[1], &rfds);
+
+    printf("MT: select()\n");
+    int rc = select(pipefds[1] + 1, &rfds, nullptr, nullptr, nullptr);
+    if (rc < 0) {
+        perror("select");
+        return 1;
+    }
+
+    printf("ok\n");
+    return 0;
+}