ci: Run tests on other platforms

Rust-simplicity supports MacOS and Windows since the latest updates.
This commit enables CI for MacOS. I would enable Windows as well, but
the install-nix-action doesn't support it.

Author: uncomputable

.github/workflows/test.yml

@@ -21,7 +21,10 @@ jobs:
 
   test-stable:
     name: Test - stable toolchain
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest]
 
     steps:
     - name: Checkout