How to test bundles in Symfony 4?

Question

symfony

When creating a module (bundle), there were difficulties in writing a unit test. Project on Symfony 4 WebSocketBundle
When running the tests on the main application, there were no problems, but when running the tests separately from the main application, they got an error about the need to set KERNEL_CLASS.
What's the best way to test?
from options:

• rewrite tests without dependency on symfony DI
  
• implement the AppKernel in the test environment.