Add test_discard.cpp

rt/test_discard.cpp

+#include <task.hpp>
+
+#include <cstdio>
+
+using namespace typon;
+
+task<int> add(int a, int b) {
+  printf("add(%d, %d)\n", a, b);
+  co_return a + b;
+}
+
+task<int> call_add_but_ignore_result(int a, int b) {
+  co_await add(a, b).discard();
+  co_return 1;
+}
+
+int main() {
+  call_add_but_ignore_result(2, 3).call_discard();
+}