Use pytest-xdist in Azure and Linux on Travis.

ci/azure-steps.yml

@@ -154,6 +154,10 @@ steps:
      where.exe python
      python --version
 
+     # Needed for running unit tests in parallel.
+     python -m pip install --upgrade pytest-xdist
+
+
      echo ""
      echo "Locating cl, rc:"
      where.exe cl

ciimage/Dockerfile

@@ -7,6 +7,7 @@ ENV DC=gdc
 RUN sed -i '/^#\sdeb-src /s/^#//' "/etc/apt/sources.list" \
 && apt-get -y update && apt-get -y upgrade \
 && apt-get -y build-dep meson \
+&& apt-get -y install python3-pytest-xdist \
 && apt-get -y install python3-pip libxml2-dev libxslt1-dev cmake libyaml-dev \
 && python3 -m pip install hotdoc codecov \
 && apt-get -y install wget unzip \