[wasm] Run Wasm.Build.Tests across .NET versions

[maraf]

We should have an automated way of ensuring that the contract between SDK and workload is working across .NET versions (eg. targeting .NET 8 apps from .NET 9 SDK).
The Wasm.Build.Tests contains "all the possible" build situations and so it is a good candidate.

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[lewing]

What parts of the contract are you thinking of specifically here?