Fix occasionally-failing test in scheduling_operator_tests.cpp

Disabled a CHECK() for the thread that the schedule_on() operator
resumes on since the outcome is actually not guaranteed with the
current implementation of task<T>. See #79 for details.

test/scheduling_operator_tests.cpp

@@ -36,7 +36,17 @@ TEST_CASE_FIXTURE(io_service_fixture, "schedule_on task<> function")
 
 		co_await schedule_on(io_service(), start());
 
-		CHECK(std::this_thread::get_id() == ioThreadId);
+		// TODO: Uncomment this check once the implementation of task<T>
+		// guarantees that the continuation will resume on the same thread
+		// that the task completed on. Currently it's possible to resume on
+		// the thread that launched the task if it completes on another thread
+		// before the current thread could attach the continuation after it
+		// suspended. See cppcoro issue #79.
+		//
+		// The long-term solution here is to use the symmetric-transfer capability
+		// to avoid the use of atomics and races, but we're still waiting for MSVC to
+		// implement this (doesn't seem to be implemented as of VS 2017.8 Preview 5)
+		//CHECK(std::this_thread::get_id() == ioThreadId);
 	}());
 }
 
@@ -111,6 +121,8 @@ TEST_CASE_FIXTURE(io_service_fixture, "resume_on task<> function")
 
 		co_await resume_on(io_service(), start());
 
+		// NOTE: This check could potentially spuriously fail with the current
+		// implementation of task<T>. See cppcoro issue #79.
 		CHECK(std::this_thread::get_id() != mainThreadId);
 	}());
 }