tests: fix building with GCC11

Fixes `error: ‘sleep_for’ is not a member of ‘std::this_thread’`

tests/threadpooltester.cpp

@@ -21,6 +21,7 @@
 
 #include "opendht/thread_pool.h"
 #include <atomic>
+#include <thread>
 
 namespace test {
 CPPUNIT_TEST_SUITE_REGISTRATION(ThreadPoolTester);