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

action.yml

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+name: jw-pkg packaging test
+description: Checkout, build and test a project organized with jw-pkg
+
+runs:
+  using: composite
+  steps:
+    - uses: ci/action-checkout@main-jw
+      with:
+        path: ${{ github.event.repository.name }}
+        working-directory: ${{ github.event.repository.name }}
+        persist-credentials: false
+    - name: Install build prerequisites
+      shell: bash
+      run: |
+        set -euxo pipefail
+        # Install from source what's publicly available. This step could be omitted.
+        (
+          export PROJECTS_DIR_REMOTE_BASE=${{ github.server_url }}
+          owner=janware
+          makefile=$PROJECTS_DIR_REMOTE_BASE/$owner/jw-pkg/raw/branch/master/make/projects-dir-minimal.mk
+          curl --create-dirs -o build-jw-foss/Makefile $makefile
+          make -C build-jw-foss pkg-install-release-deps
+          OFFLINE=true make -C build-jw-foss pkg-rebuild-reinstall
+        )
+
+    - name: Build and install package
+      shell: bash
+      run: |
+        set -ex
+        make -C ${{ github.event.repository.name }} pkg-manager-refresh # redundant
+        make -C ${{ github.event.repository.name }} pkg-install-testbuild-deps # redundant
+        make -C ${{ github.event.repository.name }} pkg-rebuild
+        make -C ${{ github.event.repository.name }} pkg-reinstall