ci: speed up CI using multiple processes with pytest-xist (#67064)

Jenkinsfile

@@ -6,7 +6,7 @@ pipeline {
     stages {
         stage('Unit Tests') {
             steps {
-                sh 'tox -rv'
+                sh 'NUMPROCESSES=8 tox -rv'
             }
             post {
                 always {

tox.ini

@@ -5,6 +5,7 @@ envlist = py3-django22-codestyle-coverage, pylint
 [testenv]
 sitepackages = true
 setenv =
+    NUMPROCESSES={env:NUMPROCESSES:1}
     SETUPTOOLS_USE_DISTUTILS=stdlib
     PYTHONPATH={toxinidir}{:}{env:PYTHONPATH:}
     PYTHONHASHSEED=0
@@ -21,6 +22,7 @@ deps =
     pytest-mock
     pytest-cov
     pytest-django
+    pytest-xdist
     pytest-freezegun
     WebTest
     mechanize
@@ -47,7 +49,7 @@ deps =
     pyzbar
     bleach
 commands =
-    py.test -v {env:COVERAGE:} --junitxml=junit-{envname}.xml {posargs:tests/}
+    py.test -v {env:COVERAGE:} --numprocesses={env:NUMPROCESSES:1} --dist loadfile --junitxml=junit-{envname}.xml {posargs:tests/}
     codestyle: pre-commit run --all-files --show-diff-on-failure
 
 [testenv:pylint]