Remove system hooks TEST endpoint

What

Follow-up from: https://gitlab.com/gitlab-org/gitlab/-/issues/376994

While adding system_hooks to the OpenApiV2 documentation, we came across the TEST endpoint where the response was returning a dummy data:

data: {
  event_name: "project_create",
  name: "Ruby",
  path: "ruby",
  project_id: 1,
  owner_name: "Someone",
  owner_email: "example@gitlabhq.com"
}

We have to ensure that endpoint works as expected.

Proposal #1

We should remove the endpoint and associated documentation because no one is using that.

• https://gitlab.com/gitlab-org/gitlab/-/blob/fe958633ff2edba506ff601166d7955e544e9c68/lib/api/system_hooks.rb#L106-L117
• https://gitlab.com/gitlab-org/gitlab/-/blob/fe958633ff2edba506ff601166d7955e544e9c68/doc/api/system_hooks.md#L127-L155

Proposal #2 (Discarded)

We should change the endpoint to match the current documentation test-system-hook. Example:

params :trigger_params do
  requires :trigger, type: String,
    values: %w[push_events tag_push_events merge_requests_events repository_update_events],
    desc: 'Set of events to be triggered against the system hook'
end


desc 'Test system hook' do
  detail 'Tests a system hook'
  success code: 200, message: 'OK'
  failure [
    { code: 400, message: 'Validation error' },
    { code: 404, message: 'Not found' },
    { code: 422, message: 'Unprocessable entity' }
  ]
  tags system_hooks_tags
end
params do
  requires :hook_id, type: Integer, desc: 'The ID of the system hook'
  use :trigger_params
end
post ":hook_id/test" do
  hook = find_hook
  TestHooks::SystemService.new(hook, current_user, params[:trigger]).execute
end

The API::Hooks::Test isn't used anywhere else, so if we're not using it anymore, we should remove it. We should update following specs:

• spec/requests/api/system_hooks_spec.rb
• spec/support/shared_examples/requests/api/hooks_shared_examples.rb

Examples:

describe "POST #{prefix}/:hook_id/test" do
  it 'tests the hook' do
    expect(WebHookService)
      .to receive(:new).with(hook, anything, String, force: false)
      .and_return(instance_double(WebHookService, execute: nil))
    expect(test_service)
      .to receive(:new).with(hook, user, String)
      .and_return(instance_double(test_service, execute: nil))

    post api(hook_uri, user)
    post api(test_hook_uri, user), params: test_params

    expect(response).to have_gitlab_http_status(:created)
  end
end

def test_hook_uri(hook_id = hook.id)
  "/hooks/#{hook_id}/test"
end
let(:test_params) do
  { trigger: 'push_events' }
end

let(:test_service) do
  TestHooks::SystemService
end