Tests/LibELF: Test loading libraries with dynamic TLS

The setup is a bit peculiar: both the definition and the use site of
these TLS variables have to be in a shared library, otherwise the linker
might relax the global-dynamic access mode to something that doesn't
require a `__tls_get_addr` call.

Tests/LibELF/TestTLS.cpp

@@ -0,0 +1,16 @@
+/*
+ * Copyright (c) 2023, Daniel Bertalan <dani@danielbertalan.dev>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibTest/TestCase.h>
+
+// When linking an executable, TLS relaxations might be relaxed to different
+// access modes than intended. Hence, the actual logic has been moved to a
+// shared library, and this executable just calls it.
+extern void run_test();
+TEST_CASE(basic)
+{
+    run_test();
+}