[Trivial] Disable compiler warnings about unused functions #7169

After #7144, some compilers emit warnings about unused functions in the scheduler tests:

  CXX      test/test_test_bitcoin-scheduler_tests.o
test/scheduler_tests.cpp:18:13: warning: function 'microTask' is not needed and will not be emitted [-Wunneeded-internal-declaration]
static void microTask(CScheduler& s, boost::mutex& mutex, int& counter, int delta, boost::chrono::system_clock::time_point rescheduleTime)
            ^
test/scheduler_tests.cpp:31:13: warning: unused function 'MicroSleep' [-Wunused-function]
static void MicroSleep(uint64_t n)
            ^
2 warnings generated.

Moving #if 0 a bit up helps to silence this warning.

TBH, the underlying issue with the test (or scheduler) should be solved instead. This is a good reminder. No need to silence a warning that points out an actual problem.

See #7144, #6540