MDL-72125 testing: Set a default idnumber when creating activities

MDL-72125 testing: Set a default idnumber when creating activities

The activity generator currently requires an idnumber when creating
activities, but this is not a requirement when creating the same
activity through the UI. The requirement comes because we want to
provide a way to refer to activities in subsequent steps.

This commit modifies the behaviour such that the generator uses the name
of the activity as the default idnumber.

This has two  main benefits:
1. it simplfies generation of activities; and
2. it makes the language used when writing behat tests much more natural.

With this change, steps will refer to the activity by its idnumber/title
in all cases, rather than sometimes by an idnumber which bears no
relevance to the title.