github-actions: Run pyocf test with pytest-xdist

Accorging to GitHub documentation ubuntu-lastest runner has 4 CPUs,
thus set xdist to use 4 workers.

Signed-off-by: Robert Baldyga <robert.baldyga@unvertical.com>

Author: robertbaldyga

.github/workflows/tests-master.yml

@@ -18,7 +18,8 @@ jobs:
       run: |
           python -m pip install --upgrade pip
           pip install pytest
+          pip install pytest-xdist
     - name: 'Build OCF library'
       run: cd tests/functional; make -j
     - name: 'Execute tests'
-      run: cd tests/functional; python3 -m pytest
+      run: cd tests/functional; python3 -m pytest -n 4

.github/workflows/tests-pr.yml

@@ -15,7 +15,8 @@ jobs:
       run: |
           python -m pip install --upgrade pip
           pip install pytest
+          pip install pytest-xdist
     - name: 'Build OCF library'
       run: cd tests/functional; make -j
     - name: 'Execute tests'
-      run: cd tests/functional; python3 -m pytest
+      run: cd tests/functional; python3 -m pytest -n 4