Why can't CircleCi find my tests?

Please see repo structure below. I want to run the tests in root/app2/tests/. I am using py.test.

After CircleCi couldn't infer test directory automatically I added a circle.yml file to the root directory but still the tests are not found. Any help greatly appreciated.

circle.yml file content:

general:
  build_dir: app2/tests

Repository structure:

root
├── circle.yml
├── app1
│   ├── xxx
│   ├── yyy
│  
└── app2
    ├── src
    ├── tests
        |-- test_module_1.py
        |-- test_module_2.py