C#: testing is disabled in CI for now

[alexcrichton]

Disabled in #1362, an example failure is https://github.com/bytecodealliance/wit-bindgen/actions/runs/17159453521/job/48684670760?pr=1360

[jsturtevant]

fyi @yowl, maybe there was an update in the runtime packages? Have you seen this in your recent work?

[yowl]

Not seen it locally yet, but that maybe becuase I've not updated my wasi-sdk

[GerardSmit]

I think CI started failing because:

• On 13 Agustus; WASI SDK was bumped to 27 in CI
• On 20 Augustus; NativeAOT-LLVM enabled -lwasi-emulated-pthread in PR runtimelab#3149.
  After that, CI started failing

This is because in WASI SDK 27, -lwasi-emulated-pthread was removed.