Ruby is tested with minitest through rake tasks.

Almost every test recreates a different environment and must run in a separate process in order to avoid to override other tests. That means that you can run a single test file (or a single spec in a test file) and it will be OK, but you cannot run a directory of test file, with a single minitest command because they will interfere with each other and will fail.

If you want to run all code tests use the :test task. For the full suite (including rubocop coverage and other checks), you can just run rake default (or just rake), or HTML_REPORTS=true rake to generate also a nice HTML report.

There is a rake task for each test file: you can get the full list of of all the test tasks (and test files that each task run) with: rake -D test_*.

If you use RubyMine or VSCode, you can find the tasks already setup in the project.

See also the E2E Test Environment.