Make runner's veryfication working again

In APIv1 we were using UpdateJob to verify if the runner exists. It was
the only method that was using Runner's token and used in special way
had no side effects (like scheduling a new job or unregisterring a
Runner).

In APIv4 we've change UpdateJob to use job's token as authentication
credentials, and that way we've removed the only endpoint that could
be used to verify if the Runner with a certain token exists in target
GitLab installation.

This commit adds `POST /api/v4/runners/verify` endpoint whose only
responsibility is to respond if Runner with posted credentials exists or
not.

lib/api/runner.rb

@@ -47,6 +47,17 @@ module API
         authenticate_runner!
         Ci::Runner.find_by_token(params[:token]).destroy
       end
+
+      desc 'Validates authentication credentials' do
+        http_codes [[200, 'Credentials are valid'], [403, 'Forbidden']]
+      end
+      params do
+        requires :token, type: String, desc: %q(Runner's authentication token)
+      end
+      post '/verify' do
+        authenticate_runner!
+        status 200
+      end
     end
 
     resource :jobs do

spec/requests/api/runner_spec.rb

@@ -152,6 +152,34 @@ describe API::Runner do
         end
       end
     end
+
+    describe 'POST /api/v4/runners/verify' do
+      let(:runner) { create(:ci_runner) }
+
+      context 'when no token is provided' do
+        it 'returns 400 error' do
+          post api('/runners/verify')
+
+          expect(response).to have_http_status :bad_request
+        end
+      end
+
+      context 'when invalid token is provided' do
+        it 'returns 403 error' do
+          post api('/runners/verify'), token: 'invalid-token'
+
+          expect(response).to have_http_status 403
+        end
+      end
+
+      context 'when valid token is provided' do
+        it 'deletes Runner' do
+          post api('/runners/verify'), token: runner.token
+
+          expect(response).to have_http_status 200
+        end
+      end
+    end
   end
 
   describe '/api/v4/jobs' do