ci: add unit tests to azure pipelines (#325)

##### SUMMARY

Enable unit testing in the CI. We started to write some unit tests for
the inventory, but they were not enforced in the CI.

tests/unit/requirements.txt

@@ -0,0 +1,2 @@
+python-dateutil
+requests

tests/utils/shippable/shippable.sh

@@ -74,7 +74,7 @@ else
 fi
 
 # STAR: HACK install dependencies
-retry pip install -r tests/integration/requirements.txt -c tests/integration/constraints.txt
+retry pip install -r tests/integration/requirements.txt -c tests/constraints.txt
 retry ansible-galaxy -vvv collection install -r tests/requirements.yml
 
 retry pip install rstcheck

tests/utils/shippable/units.sh

@@ -0,0 +1,17 @@
+#!/usr/bin/env bash
+
+set -o pipefail -eux
+
+declare -a args
+IFS='/:' read -ra args <<< "$1"
+
+python_version="${args[1]}"
+
+ansible-test env --timeout 30 --color -v
+
+# shellcheck disable=SC2086
+ansible-test units --color -v \
+    --docker default \
+    --python "$python_version" \
+    ${COVERAGE:+"$COVERAGE"} \
+    ${CHANGED:+"$CHANGED"}