Fix false allocations detection

[ig-sinicyn]

Host.BeforeMainRun(); should run before gc stats init as some host implementations (in-process, as example) may allocate during notification.

UPD: Allocation in InProcessHost is triggered by WriteLine method:

                case HostSignal.BeforeMainRun:
                    diagnoser?.BeforeMainRun(diagnoserActionParameters);
                    WriteLine(Engine.Signals.BeforeMainRun);
                    break;

[AndreyAkinshin]

@adamsitnik, could you take a look?

[adamsitnik]

good catch!