Consider using pytest-xdist for faster testing

[jakob1379]

The rather comprehensive testing is taking a long time. The testing below shows that adding the pytest-xdist plugin for pytest drastically reduces the time. The single test now fails is usually a sign that one test depends on the output of another test and should be consolidated:

❯ time pytest
=================================================================================== test session starts ===================================================================================
.
.
.
================================================================================= short test summary info =================================================================================
FAILED spacy/tests/test_cli_app.py::test_project_run - assert 1 == 0
FAILED spacy/tests/test_cli_app.py::test_project_push_pull - assert 1 == 0
============================================== 2 failed, 3611 passed, 1229 skipped, 24 xfailed, 7 xpassed, 36 warnings in 293.38s (0:04:53) ===============================================

real    4m54.536s
user    4m34.821s
sys     0m5.337s

installing pytext-xdist and running in parallel

❯ pip install pytest-xdist
Resolved 7 packages in 159ms
Prepared 2 packages in 37ms
Installed 2 packages in 1ms
 + execnet==2.1.1
 + pytest-xdist==3.8.0
❯ time pytest -n auto
=================================================================================== test session starts ===================================================================================
.
.
.
================================================================================= short test summary info =================================================================================
FAILED spacy/tests/test_cli_app.py::test_project_run - assert 1 == 0
FAILED spacy/tests/test_cli_app.py::test_project_push_pull - assert 1 == 0
============================================== 2 failed, 3611 passed, 1229 skipped, 25 xfailed, 6 xpassed, 51 warnings in 141.90s (0:02:21) ===============================================

real    2m22.511s
user    10m32.158s
sys     0m15.194s