workflows/tests: Test that bundleing and running some software works

- In light of 7944186, to avoid future
  regressions, I was inspired by the `brew services start` tests in [the
  `brew services`
  repo](https://github.com/Homebrew/homebrew-services/blob/master/.github/workflows/tests.yml)
  to make a very simple functional test that this actually installs what
  it says it does.

.github/workflows/tests.yml

@@ -43,5 +43,11 @@ jobs:
 
     - name: Run RSpec tests
       run: bundle exec rspec
+
+    - name: Test that installing works
+      run: |
+        echo 'brew "hello"' > ./Brewfile
+        brew bundle
+        hello
       
     - uses: codecov/codecov-action@51d810878be5422784e86451c0e7c14e5860ec47