[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>

Makefile

@@ -1,4 +1,4 @@
-.PHONY: manifests repos assets
+.PHONY: manifests repos assets unit-tests helm-unit-tests
 
 build-deps:
 	@command -V find docker skopeo jq gh helm > /dev/null
@@ -46,6 +46,11 @@ test:
 	make -C packages/core/testing apply
 	make -C packages/core/testing test
 
+unit-tests: helm-unit-tests
+
+helm-unit-tests:
+	hack/helm-unit-tests.sh
+
 prepare-env:
 	make -C packages/core/testing apply
 	make -C packages/core/testing prepare-cluster