Rewrite System.Text.Json stream tests to be async friendly and enable on WASM (#38663)

Rewrite System.Text.Json stream tests to be async friendly and enable on WASM (#38663)

The tests dealing are using a (De)SerializationWrapper so the same code can be used both for String and Stream types.
It does that by wrapping the async Stream serialization calls in `Task.Run().GetAwaiter().GetResult()` to turn them into sync calls.
However that doesn't work on WebAssembly since we can't wait on tasks as there's only a single thread.

To fix this inverse the wrapper so the synchronous String calls are turned into async and use normal awaits for the Stream calls.

This allows the test suite to pass on WebAssembly: `Tests run: 8349, Errors: 0, Failures: 0, Skipped: 11. Time: 475.528706s`