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/stress-truncate.cpp

@@ -0,0 +1,48 @@
+/*
+ * Copyright (c) 2021, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Random.h>
+#include <LibCore/ArgsParser.h>
+#include <fcntl.h>
+#include <inttypes.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+int main(int argc, char** argv)
+{
+    const char* target = nullptr;
+    int max_file_size = 1024 * 1024;
+    int count = 1024;
+
+    Core::ArgsParser args_parser;
+    args_parser.add_option(max_file_size, "Maximum file size to generate", "max-size", 's', "size");
+    args_parser.add_option(count, "Number of truncations to run", "number", 'n', "number");
+    args_parser.add_positional_argument(target, "Target file path", "target");
+    args_parser.parse(argc, argv);
+
+    int fd = creat(target, 0666);
+    if (fd < 0) {
+        perror("Couldn't create target file");
+        return EXIT_FAILURE;
+    }
+    close(fd);
+
+    for (int i = 0; i < count; i++) {
+        auto new_file_size = AK::get_random<uint64_t>() % (max_file_size + 1);
+        printf("(%d/%d)\tTruncating to %" PRIu64 " bytes...\n", i + 1, count, new_file_size);
+        if (truncate(target, new_file_size) < 0) {
+            perror("Couldn't truncate target file");
+            return EXIT_FAILURE;
+        }
+    }
+
+    if (unlink(target) < 0) {
+        perror("Couldn't remove target file");
+        return EXIT_FAILURE;
+    }
+
+    return EXIT_SUCCESS;
+}