kokoro: not all tests being run

Debugging #1525, I discovered that the tests being based on the logic in [.toys/kokoro-ci.rb](https://github.com/GoogleCloudPlatform/ruby-docs-samples/blob/main/.toys/kokoro-ci.rb#L140) may only include Gemfiles one and two levels deep.

This means that there are a number of tests in deeper nested directories that aren't being run.

My debugging in changing the `Gemfile` glob to `Dir.glob("*/**/Gemfile").each` (ignoring the top level Gemfile) gives more tests to execute (but also more failures).

Internal details b/392885740#comment6