[ci,dx] Add unit tests for cozy-lib

## What this PR does

The cozy-lib library package got complicated enough to warrant its own
unit tests. Since unit tests are a "good thing" (tm), a somewhat generic
framework for running all kinds of unit tests was introduced into the CI
pipeline and Makefile targets. For now all it runs is `make test`
against the `packages/{library,apps,system,extra}/*` directories,
wherever a `test` target is present in the Makefile, and for now this is
only for the `cozy-lib` Helm library chart.

### Release note

```release-note
[ci,dx] Introduce a scaffold for running unit tests locally and in CI
and add the first unit tests for the cozy-lib helper Helm chart.
```

Signed-off-by: Timofei Larkin <lllamnyp@gmail.com>

.github/workflows/pull-requests.yaml

@@ -33,6 +33,9 @@ jobs:
           fetch-depth: 0
           fetch-tags: true
 
+      - name: Run unit tests
+        run: make unit-tests
+
       - name: Set up Docker config
         run: |
           if [ -d ~/.docker ]; then