ci: add stimulus_reflex to downstream suite (#3027)

**What problem is this PR intended to solve?**

Closes #3024, adding stimulus_reflex to the downstream CI suite.

cc @marcoroth

.github/workflows/downstream.yml

@@ -51,6 +51,9 @@ jobs:
           - url: https://github.com/sparklemotion/mechanize
             name: mechanize
             command: "bundle exec rake test"
+          - url: https://github.com/stimulusreflex/stimulus_reflex
+            name: stimulus_reflex
+            command: "bundle exec rake test"
           # - url: https://github.com/instructure/nokogiri-xmlsec-instructure
           #   name: nokogiri-xmlsec-instructure
           #   precommand: "apt install -y libxmlsec1-dev"