README: Add explanation how to run the tests

[petervandenabeele]

On a Linux box with conda, this could be an explanation on how to get pytest running.

/home/peter_v/anaconda3/bin/python -m pip install --upgrade pip  # to avoid warnings about spyder 4.1.5 versions
make
sudo apt install nodejs  # to avoid massive warnings about RuntimeError: generator didn't stop after throw()

$ poetry run pytest
========================================== test session starts ===========================================
platform linux -- Python 3.8.2, pytest-6.2.2, py-1.10.0, pluggy-0.13.1
rootdir: /home/peter_v/data/github/vincentclaes/datajob
collected 16 items                                                                                       

datajob_tests/test_datajob_context.py .                                                            [  6%]
datajob_tests/test_datajob_stack.py ....                                                           [ 31%]
datajob_tests/datajob_cli_tests/test_datajob_deploy.py .......                                     [ 75%]
datajob_tests/datajob_cli_tests/test_datajob_execute.py .                                          [ 81%]
datajob_tests/glue/test_glue_job.py .                                                              [ 87%]
datajob_tests/stepfunctions/test_stepfunctions_workflow.py ..                                      [100%]

=========================================== 16 passed in 5.62s ===========================================