[browser][mt] Stop using ThreadlessXunitTestRunner for MT unit tests

[pavelsavara]

On wasm/browser we use Xharness and it's ThreadlessXunitTestRunner because until now, we didn't have way how to run multiple threads and how to synchronously Task.Wait.

To make sure that our MT tests are closer to normal MT execution, we should try to stop using ThreadlessXunitTestRunner and see what happens when we use the same as other OSes.

• The non-MT build should still be using it. So this needs to be new xharness parameter.
• WasmTestRunner is part of the machinery
• We still need to proxy console from the browser.
• It may mean that we will run tests in parallel on threads.
• It will uncover new issues, which we need to mark ActiveIssue

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Issue Details

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Author:	pavelsavara
Assignees:	pavelsavara
Labels:	arch-wasm, area-Infrastructure-mono, os-browser
Milestone:	8.0.0

[pavelsavara]

To make local experiments easier, you can build Xharness locally and then
set XHARNESS_CLI_PATH=c:\Dev\xharness\artifacts\bin\Microsoft.DotNet.XHarness.CLI\Release\net6.0\Microsoft.DotNet.XHarness.CLI.dll

runtime tests will start using your local build.