Ability to run E2E tests on WebAssembly multithreaded runtime

[SteveSandersonMS]

Fixes #53435

Provides an easy way to have all the E2E tests run on the WebAssembly multithreaded runtime by setting a single MSBuild property in src/Components/test/testassets/Directory.Build.props.

  <PropertyGroup>
    <!--
      This controls whether the WebAssembly E2E tests run with the multithreaded runtime or not.
      It also implicitly sets an assembly metadata attribute so the test servers will serve the COOP
      headers if and only if this flag is set. Note that until https://github.com/dotnet/runtime/issues/98502
      is fixed, you will have to delete artifacts\obj directories after changing this flag.
    -->
    <!--<WasmEnableThreads>true</WasmEnableThreads>-->
  </PropertyGroup>

It's commented out because we don't actually want to enable this by default, because:

1. This is not the common case
2. Currently, many of the E2E tests will fail until we implement Make Blazor WebAssembly work on multithreaded runtime #54365

The purpose of this PR is to simplify collaboration around implementing proper support for WebAssembly multithreading.

[javiercn]

Looks great!

[javiercn]

@SteveSandersonMS Do we want to add an extra CI config to run this leg too?

[SteveSandersonMS]

Do we want to add an extra CI config to run this leg too?

I'd say not until the tests are at least nearly all passing. Right now it would run for hours probably because so many fail, and failed tests take a long time because of all the retries. So it would harm the overall build times for everyone.