Create shared examples and helpers to test EventStore

Have a consistent way of testing publishing and consumption
of events.

app/events/ci/pipeline_created_event.rb

@@ -5,6 +5,7 @@ module Ci
     def schema
       {
         'type' => 'object',
+        'required' => ['pipeline_id'],
         'properties' => {
           'pipeline_id' => { 'type' => 'integer' }
         }

doc/development/event_store.md

@@ -293,8 +293,7 @@ in the `handle_event` method of the subscriber worker.
 
 ## Testing
 
-A publisher doesn't must care what subscribed to the event being published. The publisher's
-responsibility is to ensure that the event is published correctly.
+The publisher's responsibility is to ensure that the event is published correctly.
 
 To test that an event has been published correctly, we can use the RSpec matcher `:publish_event`:
 

spec/events/ci/pipeline_created_event_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Ci::PipelineCreatedEvent do
+  using RSpec::Parameterized::TableSyntax
+
+  where(:data, :valid) do
+    { pipeline_id: 1 }      | true
+    { pipeline_id: nil }    | false
+    { pipeline_id: "test" } | false
+    {}                      | false
+    { job_id: 1 }           | false
+  end
+
+  with_them do
+    let(:event) { described_class.new(data: data) }
+
+    it 'validates the data according to the schema' do
+      if valid
+        expect { event }.not_to raise_error
+      else
+        expect { event }.to raise_error(Gitlab::EventStore::InvalidEvent)
+      end
+    end
+  end
+end