CI: run tests also on macOS and Windows

.github/workflows/tests.yml

@@ -7,8 +7,14 @@ on:
 jobs:
   tests:
     name: Tests
-    runs-on: ubuntu-latest
-
+      strategy:
+        fail-fast: false # if tests for one version fail, continue with the rest
+        matrix:
+          # run on all main platforms to test platform-specific code, if present
+          # (e.g. webkit's WebCrypto API implementation is different in macOS vs Linux)
+          runner: ['ubuntu.latest', 'macos-latest', 'window.latest']
+    runs-on: ${{ matrix.runner }} 
+
     steps:
       - uses: actions/checkout@v3
       - uses: actions/setup-node@v3