Fix import error of tests.core
Fix import error of tests.core

```
======================================================================
ERROR: test_vision_sensors (unittest.loader._FailedTest)
----------------------------------------------------------------------
ImportError: Failed to import test module: test_vision_sensors
Traceback (most recent call last):
  File "/home/wkentaro/xxxx/.anaconda3/lib/python3.7/unittest/loader.py", line 436, in _find_test_path
    module = self._get_module_from_name(name)
  File "/home/wkentaro/xxxx/.anaconda3/lib/python3.7/unittest/loader.py", line 377, in _get_module_from_name
    __import__(name)
  File "/home/wkentaro/xxxx/src/pyrep/tests/test_vision_sensors.py", line 2, in <module>
    from tests.core import TestCore
ModuleNotFoundError: No module named 'tests.core'
```

stepjam/PyRep,stepjam/PyRep