Add test for verifying that each event definition file has a history table entry

[magnusbaeck]

Description

We should add a test (pytest-based, preferably) that verifies that each definitions/Eiffel*Event/x.y.z.yml has an object in its _history array with the version key set to x.y.z.

Motivation

Two recent PRs (#322 and #350) have added new versions of events without adding the required entry in the history table. That should've been caught by a test.

Exemplification

See aforementioned PRs.

Benefits

Less burden on reviewers, quicker review times, and decreased risk of errors sneaking in.

Possible Drawbacks

None.