Add a test for #22010

The observed faulty state transition is probably hard to test,
because we need to hook into internal states to observe them.
Namely this:

    07:30:16 | Build#ruby-2.2 enqueue: created -> pending
    07:30:16 | Pipeline#32    enqueue: created -> pending
    07:30:16 | Build#ruby-2.3 enqueue: created -> pending
    07:30:16 | Build#ruby-2.2     run: pending -> running
    07:30:16 | Pipeline#32        run: pending -> running
    07:30:29 | Build#ruby-2.2    drop: running -> failed
    07:30:29 | Pipeline#32        run: running -> running
    07:30:29 | Build#ruby-2.3     run: pending -> running
    07:30:30 | Pipeline#32        run: running -> running
    07:30:57 | Build#gem:build   skip: created -> skipped
    07:30:57 | Pipeline#32       drop: running -> failed
    07:30:57 | Build#gem:release skip: created -> skipped
    07:30:57 | Pipeline#32       drop:  failed -> failed
    07:30:57 | Build#ruby-2.3    drop: running -> failed
    07:30:57 | Pipeline#32       drop: running -> failed
                                       ^^^ Should be failed -> failed

However, the consequence of this, executing hooks twice would be
easy enough to observe. So we could at least test against this.
Keep in mind that if we ever changed how we execute the hooks
this won't be testing against faulty state transition.

spec/models/ci/pipeline_spec.rb

@@ -373,8 +373,8 @@ describe Ci::Pipeline, models: true do
   end
 
   describe '#execute_hooks' do
-    let!(:build_a) { create_build('a') }
-    let!(:build_b) { create_build('b') }
+    let!(:build_a) { create_build('a', 0) }
+    let!(:build_b) { create_build('b', 1) }
 
     let!(:hook) do
       create(:project_hook, project: project, pipeline_events: enabled)
@@ -427,6 +427,16 @@ describe Ci::Pipeline, models: true do
           end
         end
 
+        context 'when stage one failed' do
+          before do
+            build_a.drop
+          end
+
+          it 'receive a failed event once' do
+            expect(WebMock).to have_requested_pipeline_hook('failed').once
+          end
+        end
+
         def have_requested_pipeline_hook(status)
           have_requested(:post, hook.url).with do |req|
             json_body = JSON.parse(req.body)
@@ -450,8 +460,12 @@ describe Ci::Pipeline, models: true do
       end
     end
 
-    def create_build(name)
-      create(:ci_build, :created, pipeline: pipeline, name: name)
+    def create_build(name, stage_idx)
+      create(:ci_build,
+             :created,
+             pipeline: pipeline,
+             name: name,
+             stage_idx: stage_idx)
     end
   end
 end