Enhance scenario trigger interface

[yuriyyakym]

Right now the only way to trigger a scenario with multiple AwaiEvents's/promises is to have a trigger callback and use Promise.race, like so:

scenario(
  () => Promise.race([
    action1.events.invoked,
    delay(1000),
  ]),
  () => { /* ... */ }
);

It should also be possible to pass an array or AwaiEvents:

scenario(
  [action1.events.invoked, state1.events.changed],
  () => { /* ... */ }
);