tests: fix building with GCC11

[ArchangeGabriel]

While building on Arch, we had:

/build/opendht/src/opendht-2.3.1/tests/threadpooltester.cpp: In member function ‘void test::ThreadPoolTester::testThreadPool()’:
/build/opendht/src/opendht-2.3.1/tests/threadpooltester.cpp:47:27: error: ‘sleep_for’ is not a member of ‘std::this_thread’
   47 |         std::this_thread::sleep_for(std::chrono::milliseconds(10));
      |                           ^~~~~~~~~
/build/opendht/src/opendht-2.3.1/tests/threadpooltester.cpp: In member function ‘void test::ThreadPoolTester::testExecutor()’:
/build/opendht/src/opendht-2.3.1/tests/threadpooltester.cpp:76:27: error: ‘sleep_for’ is not a member of ‘std::this_thread’
   76 |         std::this_thread::sleep_for(std::chrono::milliseconds(10));
      |                           ^~~~~~~~~

[ArchangeGabriel]

@aberaud Anything wrong with this PR?