test: Add scheduler test coverage

[barton2526]

This adds a test case for the scheduler from upstream. The goal of this PR was initially to port bitcoin/bitcoin#18285, which adds a test case to catch bugs like this one #1887. While working I discovered we didn't test the scheduler at all, so let's get more coverage.

Ref: bitcoin/bitcoin@fa6bad2

Optional other tests include:

BOOST_AUTO_TEST_CASE(manythreads)

    // Stress test: hundreds of microsecond-scheduled tasks,
    // serviced by 10 threads.
    //
    // So... ten shared counters, which if all the tasks execute
    // properly will sum to the number of tasks done.
    // Each task adds or subtracts a random amount from one of the
    // counters, and then schedules another task 0-1000
    // microseconds in the future to subtract or add from
    // the counter -random_amount+1, so in the end the shared
    // counters should sum to the number of initial tasks performed.
...


 BOOST_AUTO_TEST_CASE(singlethreadedscheduler_ordered)

    // each queue should be well ordered with respect to itself but not other queues
    SingleThreadedSchedulerClient queue1(&scheduler);
    SingleThreadedSchedulerClient queue2(&scheduler);

    // create more threads than queues
    // if the queues only permit execution of one task at once then
    // the extra threads should effectively be doing nothing
    // if they don't we'll get out of order behaviour
...

[barton2526]

The PR compiles, but the new test case is failing in an obscure way.

   Running 413 test cases...
  unknown location(0): fatal error: in "scheduler_tests/wait_until_past": Throw location unknown (consider using BOOST_THROW_EXCEPTION)
  Dynamic exception type: boost::wrapexcept<boost::condition_error>
  std::exception::what: boost::condition_variable::do_wait_until failed in pthread_cond_timedwait: Invalid argument
  
  test/scheduler_tests.cpp(38): last checkpoint
  
  *** 1 failure is detected in the test module "Gridcoin Test Suite"
  FAIL test/test_gridcoin (exit status: 201)

It also appears to have passed on Windows 32 bit...?

Perhaps it would be better to take boost out of the scheduler?
bitcoin/bitcoin@d0ebd93