add FIXME to hybrid burn testing

tests/test_hybrid_burns.cpp

@@ -751,6 +751,7 @@ TEST_CASE("Continuous mode transitions during burns", "[hybrid][burns][continuou
     INFO("Expected energy change: " << expected_energy_change << " J/kg");
 
     REQUIRE(fabs(energy_change) > 0);
+    // FIXME: not comparing expected_energy_change
 
     destroy_simulation(sim);
 }