Enable full test suite for Wasm with main snapshots

[MaxDesiatov]

We should use swift test as wasm_sdk_build_command in pull_request.yml to actually run the tests. Previously only the library itself was built but not tested, because released versions of Swift that support Wasm didn't have working swift test when using non-embedded Swift SDK for Wasm.

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.