Workaround for CI errors: Run tests single-threaded on macOS

Author: phil-opp

.github/workflows/ci.yml

@@ -66,6 +66,7 @@ jobs:
         uses: actions-rs/cargo@v1
         with:
           command: test
+          args: "-- --test-threads 1"
 
       - name: "Example: `basic`"
         working-directory: examples/basic