Ensure CI files are valid YAML

Doesn't tell us they are fully valid, but should catch basic mistakes.

Author: byroot

railties/test/generators/app_generator_test.rb

@@ -655,8 +655,16 @@ def test_both_brakeman_and_rubocop_binstubs_are_skipped_if_required
 
   def test_inclusion_of_ci_files
     run_generator
-    assert_file ".github/workflows/ci.yml"
-    assert_file ".github/dependabot.yml"
+    assert_file ".github/workflows/ci.yml" do |yaml|
+      assert_nothing_raised do
+        YAML.load(yaml)
+      end
+    end
+    assert_file ".github/dependabot.yml" do |yaml|
+      assert_nothing_raised do
+        YAML.load(yaml)
+      end
+    end
   end
 
   def test_ci_files_are_skipped_if_required