Add pytest-xdist plugin to speed up tests

Running locally on my Mac:
- pytest: 1min 20sec
- pytest -n auto: 15sec

https://pypi.org/project/pytest-xdist/

Author: bcantoni

.github/workflows/run-tests.yml

@@ -48,7 +48,7 @@ jobs:
     - name: Test with pytest
       if: always()
       run: |
-        pytest test
+        pytest test -n auto
 
     - name: Test build
       if: always()

pyproject.toml

@@ -33,7 +33,7 @@ repository = "https://github.com/tableau/server-client-python"
 
 [project.optional-dependencies]
 test = ["black==24.8", "build", "mypy==1.4", "pytest>=7.0", "pytest-cov", "pytest-subtests",
-    "requests-mock>=1.0,<2.0"]
+    "pytest-xdist", "requests-mock>=1.0,<2.0"]
 
 [tool.black]
 line-length = 120