Cannot register mock activity with string name on test workflow environment

[cretz]

Expected Behavior

Should be able to call OnActivity("string-name" without having registered it first

Actual Behavior

We check whether it was registered first. There are activities defined in different languages that do not have a Go function to have been registered.

Stop requiring activities to be registered before mocking them.

[cretz]

We may have to require registration first, but we should at least document that we require registration first.