Add Test Coverage

Test coverage report with [spm-lcov-action](https://github.com/maxep/spm-lcov-action)

.github/workflows/main.yml

@@ -17,3 +17,4 @@ jobs:
       run: swift build
     - name: Run tests
       run: swift test --enable-code-coverage
+    - uses: maxep/spm-lcov-action@0.2.1