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

Start by reading .toys/kokoro-ci.rb around line 140 and trace how Gemfiles are discovered and passed to the test runner. Compare the current glob with the proposed recursive glob, then run the relevant Kokoro test command. Done means deeper nested Gemfiles are included without unintentionally selecting the top-level Gemfile.

Vom Indexierungsmodell aus dem Issue-Text verfasst.