Testcase run twice when import a class with testcase in python file.

**Issue description**

I try to import a class with testcase in my python file as follows.

```python
from tempest.api.network.xxx import xxxTest
```

But when executing cmd `stestr run` , the testcase in xxxTest will run ***twice***.

**Expected behavior and actual behavior**
when executing cmd `stestr run` , the testcase in xxxTest will run ***once***.


