Discussion on possible implementation strategy:

we know we need single threaded xunit runner (from xharness)
ideally we would be able to use dotnet test to run library tests
that dotnet test and dotnet run host needs to understand cross platform hosting
A) with NodeJS
B) with HTTP DevServer & Chrome

Aaron said that they prefer NodeJS because they don't want to suffer the CI pain of broken infra. And browser/HTTP troubles.
But the actual product is the browser, so we need to have B) too.

For B) we use HTTP server from xharness at the moment.
That already solves:

proper HTTP headers
streaming console log from the browser
waiting for the unit test to exit/abort
collecting the XML test results file
passing proper CLI arguments to V8, NodeJs

Downside of xharness is that it doesn't give public facing dotnet test

We also already have implementation of dotnet run in src\mono\wasm\host\. But that doesn't implement xunit AFAIK.

For B) because of async nature of HTTP fetch our WASM SDK needs to produce manifest dotnet.boot.js which contains list of all resources. This makes in-tree lib testing dependent on WASM SDK.
It's probably a good thing to use the same WASM SDK as the customers would.

For A) we can make extra effort to avoid dependency on the manifest, because on NodeJs we can use host file system. With that we could make the node dotnet.js run to behave like dotnet run. We could let the VM PAL to load DLLs from disk synchronously.
There would be 2 possible implementation strategies.

A1) Start emscripten/posix emulator and let almost normal corerun to explore the filesystem.
A2) Implement searching of the host file system in dotnet.js via Node's fs APIs. In order to create the manifest of in-flight.

All that said, NodeJS is not the target platform. And I'm not clear if the investment into A) is worth it.

cc @lewing @akoeplinger @maraf @AaronRobinsonMSFT

[browser][coreCLR] use WasmAppHost -> WasmTestRunner to run library tests #120208

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