(maint) Add specs to gha pr_testing workflow

Author: jpartlow

.github/workflows/pr_testing.yaml

@@ -42,6 +42,17 @@ jobs:
       - name: Check whether REFERENCE.md has uncommitted changes
         run: git diff --exit-code REFERENCE.md
 
+  specs:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: '3.3'
+          bundler-cache: true
+      - name: Run RSpec tests
+        run: bundle exec rspec spec
+
   test-install-task-on-ubuntu:
     strategy:
       matrix: