The followng works for me, I was having a major headache with this, turns out I had a "ADD_LATENT_AUTOMATION_COMMAND(FWaitForMapToLoadCommand());"
at the start of my test, which will never return in an editor context.
#if WITH_EDITOR
if(TestWorld->IsEditorWorld())
{
UE_LOG(LOGMYTESTS, Log, TEXT("Looks like we are running in editor"));
ADD_LATENT_AUTOMATION_COMMAND(FEditorLoadMap(expectedLevelName));
ADD_LATENT_AUTOMATION_COMMAND(FStartPIECommand(true));
}