Cirrus CI: Run unit tests as a non-privileged user.

Tests involving file access control fail systematically if the test suite is
run as root (which happens in Cirrus CI's Docker containers).  Since the
official Python containers are built on top of Debian, we might be able to run
using the UNIX standard user `bin` without creating an ad-hoc test user.

.cirrus.yml

@@ -62,13 +62,14 @@ task:
     # for testing.  The `python` official Docker image is built on top of
     # Debian, so use `apt` to install it.
     apt-get update
-    apt-get install -y time
+    apt-get install -y sudo time
   install_script: |
     pip install -e '.[daemon,ec2,openstack,optimizer]'
   test_script: |
     set -e
     pip install 'pytest>=4.1' 'pytest-cov' 'pytest-coverage' 'mock' 'tox' 'codecov'
-    pytest -v --cov=gc3libs --cov-branch --ignore install.py
+    # run tests as a non-privileged user, otherwise access control tests fail
+    sudo -u bin pytest -v --cov=gc3libs --cov-branch --ignore install.py
     codecov
     echo "OK: GC3Pie's unit tests successfully performed."
     exit 0