When we port the `Event` class to Rust, the constructor will check for
the existence of required fields. To support that, we tidy up the test
code where we construct fake events to add all the required fields.
There should be no behavioural changes.
Review commit-by-commit.
During the migration the automated script to update the copyright
headers accidentally got rid of some of the existing copyright lines.
Reinstate them.
Erik Johnston