workflow: add a test job to ci.yml

Add a job to run the ziglings unit tests, using `zig build test`.

.github/workflows/ci.yml

@@ -23,3 +23,19 @@ jobs:
 
     - name: Check compatibility with old Zig compilers
       run: ci/compat.sh
+  test:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, windows-latest, macos-latest]
+    runs-on: ${{ matrix.os }}
+    timeout-minutes: 60
+    steps:
+    - uses: actions/checkout@v3
+
+    - name: Setup Zig
+      uses: goto-bus-stop/setup-zig@v2
+      with:
+        version: master
+
+    - name: Unit Test
+      run: zig build test