Allow test objects without trigger functions for remote triggers / MVT split tests

[kingo55]

There are a few cases where we wouldn't need a trigger function for experiments e.g.:

• MVTs where we might activate two experiments at the same time from the one trigger
• Remotely triggered experiments (i.e. tigger is outside of the experiment)

However we fire an error when a testObject doesn't have a trigger function. This was originally by design, but it's probably worth revisiting in case we build more experiments like the above.

[allmywant]

@kingo55 I think we can remove that check and add another check beore running this.options.trigger(this);.

[kingo55]

addressed by #77