kokoro: not all tests being run

[glasnt]

Debugging #1525, I discovered that the tests being based on the logic in .toys/kokoro-ci.rb 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