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/pthread-cond-timedwait-example.cpp

@@ -0,0 +1,76 @@
+/*
+ * Copyright (c) 2018-2020, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <cassert>
+#include <cstring>
+#include <ctime>
+#include <errno.h>
+#include <pthread.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+struct worker_t {
+    const char* name;
+    int count;
+    pthread_t thread;
+    pthread_mutex_t lock;
+    pthread_cond_t cond;
+    long int wait_time;
+};
+
+static void* run_worker(void* args)
+{
+    struct timespec time_to_wait = { 0, 0 };
+    worker_t* worker = (worker_t*)args;
+    worker->count = 0;
+
+    while (worker->count < 25) {
+        time_to_wait.tv_sec = time(nullptr) + worker->wait_time;
+        pthread_mutex_lock(&worker->lock);
+        int rc = pthread_cond_timedwait(&worker->cond, &worker->lock, &time_to_wait);
+
+        // Validate return code is always timed out.
+        assert(rc == -1);
+        assert(errno == ETIMEDOUT);
+
+        worker->count++;
+        printf("Increase worker[%s] count to [%d]\n", worker->name, worker->count);
+        pthread_mutex_unlock(&worker->lock);
+    }
+
+    return nullptr;
+}
+
+static void init_worker(worker_t* worker, const char* name, long int wait_time)
+{
+    worker->name = name;
+    worker->wait_time = wait_time;
+
+    pthread_attr_t attr;
+    pthread_attr_init(&attr);
+    pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
+
+    pthread_mutex_init(&worker->lock, nullptr);
+    pthread_cond_init(&worker->cond, nullptr);
+    pthread_create(&worker->thread, &attr, &run_worker, (void*)worker);
+
+    pthread_attr_destroy(&attr);
+}
+
+int main()
+{
+    worker_t worker_a;
+    init_worker(&worker_a, "A", 2L);
+
+    worker_t worker_b;
+    init_worker(&worker_b, "B", 4L);
+
+    pthread_join(worker_a.thread, nullptr);
+    pthread_join(worker_b.thread, nullptr);
+
+    return EXIT_SUCCESS;
+}