ci: use pytest-xdist for parallel testing

.github/workflows/tests.yml

@@ -52,6 +52,8 @@ jobs:
       uses: actions/setup-python@v2
       with:
         python-version: 2.7
+    - name: Install pytest-xdist
+      run: pip install pytest-xdist
     - name: Install capa
       run: pip install -e .[dev]
     - name: Run tests