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/LibC/TestLibCExec.cpp

@@ -0,0 +1,25 @@
+/*
+ * Copyright (c) 2018-2020, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibTest/TestCase.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <unistd.h>
+
+TEST_CASE(exec_should_not_search_current_directory)
+{
+    int fd = open("hax", O_CREAT | O_RDWR, 0755);
+    ftruncate(fd, 0);
+    close(fd);
+
+    int rc = execlp("hax", "hax", nullptr);
+    int saved_errno = errno;
+    perror("execlp");
+    unlink("hax");
+
+    EXPECT_EQ(rc, -1);
+    EXPECT_NE(saved_errno, ENOEXEC);
+}