Add action.yml and reusable.yaml.

This action allows to test packaging of projects compatible with the jw-pkg layout.
It attempts the following steps:
- Clones, builds and installs jw-pkg from source
- Clones the package under test from Git
- Installs its build prerequisites
- Builds, packages and installs it
Additionally, a reusable workflow .github/workflows/reusable.yaml is added to invoke it for all supported distributions, which currently are Kali Linux and OpenSUSE.
Signed-off-by: Jan Lindemann <jan@janware.com>

.github/workflows/reusable.yaml

@@ -0,0 +1,20 @@
+name: Reusable packaging test job
+
+on:
+  workflow_call:
+
+jobs:
+  test:
+    runs-on: public-ci
+    strategy:
+      fail-fast: false
+      matrix:
+        distro:
+          - { image: "registry.janware.com/public/ci/runner/opensuse/tumbleweed:latest", name: "OpenSUSE Tumbleweed" }
+          - { image: "registry.janware.com/public/ci/runner/kali/rolling:latest",        name: "Kali Linux" }
+
+    container:
+      image: ${{ matrix.distro.image }}
+
+    steps:
+      - uses: ci/action-build-package@master