Add missing compile item in System.Net.WebSockets.Client.Wasm.Tests.csproj

[ViktorHofer]

Fixes:

C:\git\runtime3> .\dotnet.cmd build src\libraries\System.Net.WebSockets.Client\tests\wasm\System.Net.WebSockets.Client.Wasm.Tests.csproj /bl

  Determining projects to restore...
  All projects are up-to-date for restore.
  TestUtilities -> C:\git\runtime3\artifacts\bin\TestUtilities\Debug\net8.0\TestUtilities.dll
C:\git\runtime3\src\libraries\Common\tests\System\Net\Http\GenericLoopbackServer.cs(15,23): error CS0234: The type or namespace name 'Functional' does not exist in the
 namespace 'System.Net.Http' (are you missing an assembly reference?) [C:\git\runtime3\src\libraries\System.Net.WebSockets.Client\tests\wasm\System.Net.WebSockets.Clie
nt.Wasm.Tests.csproj]
C:\git\runtime3\src\libraries\Common\tests\System\Net\Http\GenericLoopbackServer.cs(26,68): error CS0103: The name 'TestHelper' does not exist in the current context [
C:\git\runtime3\src\libraries\System.Net.WebSockets.Client\tests\wasm\System.Net.WebSockets.Client.Wasm.Tests.csproj]

This wasn't caught before because there is no CI validation for building tests for all configurations: #106474

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[ViktorHofer]

Merging as this change isn't exercised in CI anyway atm. Will be fixed with #106474