SW-1427 improve README (mrbeam#1534)

Author: irlaec

README.md

@@ -35,10 +35,16 @@ Access from the browser:
 ## Running unit tests
 Create a Python 2 virtual environment and activate it:
 ```shell
-virtualenv -p /usr/bin/python2.7 tests_req
+virtualenv -p /usr/bin/python2.7 tests_venv
+source tests_venv/bin/activate
+```
+
+Install test requirements:
+```shell
+pip install -r test-requirements.txt
 ```
 
 Run the unit tests directly from **Pycharm** or from the command line:
 ```shell
-python -m pytest paht/to/MrBeamPlugin/tests/
+python -m pytest tests/
 ```