adding missing test

tests/task_cancel.cpp

@@ -0,0 +1,16 @@
+#include <fc/thread/thread.hpp>
+#include <fc/log/logger.hpp>
+#include <fc/exception/exception.hpp>
+
+int main( int argc, char** argv )
+{
+   try {
+       auto result = fc::schedule( [=]() { ilog( "hello world" ); }, fc::time_point::now() + fc::seconds(3) );
+       result.cancel();
+       result.wait();
+   } 
+   catch ( const fc::exception& e )
+   {
+      wlog( "${e}", ("e",e.to_detail_string() ) );
+   }
+}