Build and run tests for Wasm in pull_request.yml

[MaxDesiatov]

We only build the package for Wasm but don't test it. Unfortunately, test for Wasm are not directly supported by swift test yet, but we can run wasmkit manually to work around this.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• [N/A] I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• [N/A] I've updated the documentation if necessary.