Fix randomly failing scheduler_tests test

Summary
---

This closes issue Bitcoin-ABC#266. The root cause is that the original test author
was misusing threads and/or synchronization primitives. There is no
guarantee that the first main thread runs before the last scheduled
task. As such, sometimes, the last task runs and writes "42" into the
`counter` before the `BOOST_CHECK_EQUAL(counter, 0)` line gets a chance
to be evaluated in the main thread.

The fix is to not rely on undefined behavior here and instead do things
properly.  We just save the counter var to a second atomic and check
everything at the end after the two subordinate tasks have all
definitely finished running.

Test Plan
---

- `ninja all check`
- Try and reproduce the issue in Bitcoin-ABC#266 as described. If you can reproduce
  it against master but not here, then you can be happy this is fixed.

An alternative way to test if you *can't* reproduce the failure against master
is to:

1. `git checkout master`
2. Edit `src/test/scheduler_tests.cpp` and insert a
   `std::this_thread::sleep_for(std::chrono::milliseconds(21));` right
*before* the line in the `schedule_every` test that does
`BOOST_CHECK_EQUAL(counter, 0);`. This will reproduce the failure every
time.
3. `ninja test_bitcoin && src/test/test_bitcoin -t scheduler_tests`
4. `git checkout THIS_MR_BRANCH`
5. Add the sleep call in approximately the same place and do steps (2 & 3)
   again.  You should never get a failure now.

src/test/scheduler_tests.cpp

@@ -138,11 +138,11 @@ BOOST_AUTO_TEST_CASE(schedule_every) {
     CScheduler scheduler;
 
     std::condition_variable cvar;
-    std::atomic<int> counter{15};
+    std::atomic<int> counter{15}, savedCounter{-1};
     std::atomic<bool> keepRunning{true};
 
     scheduler.scheduleEvery(
-        [&keepRunning, &cvar, &counter, &scheduler]() {
+        [&keepRunning, &cvar, &counter, &savedCounter, &scheduler]() {
             assert(counter > 0);
             cvar.notify_all();
             if (--counter > 0) {
@@ -160,7 +160,7 @@ BOOST_AUTO_TEST_CASE(schedule_every) {
 
             // We set the counter to some magic value to check the scheduler
             // empty its queue properly after 120ms.
-            scheduler.scheduleFromNow([&counter]() { counter = 42; }, 120);
+            scheduler.scheduleFromNow([&counter, &savedCounter]() { savedCounter = counter.exchange(42); }, 120);
             return false;
         },
         5);
@@ -176,10 +176,10 @@ BOOST_AUTO_TEST_CASE(schedule_every) {
         BOOST_CHECK(counter >= 0);
     }
 
-    BOOST_CHECK_EQUAL(counter, 0);
     scheduler.stop(true);
     schedulerThread.join();
     BOOST_CHECK_EQUAL(counter, 42);
+    BOOST_CHECK_EQUAL(savedCounter, 0);
 }
 
 BOOST_AUTO_TEST_CASE(singlethreadedscheduler_ordered) {