fix(xunit): support netstandard2.0

[mxschmitt]

ModuleInitializer was something which was not supported in older .NET frameworks. This patch starts listening on the constructor instead and resets the TestOK status in TestInitialize.

Background: xUnit 2 does not provide a reliable way of determining if a test was passing or failing and reading it's status in the AfterEach. xUnit 3 allows to read its status which we'll use once its published. We want to support xUnit 2 as well.

#2977

[mxschmitt]

drive-by: Similar to 7f3b888.

[kblok]

How about targeting netstandard instead?

[campersau]

xunit.runner.visualstudio and Microsoft.NET.Test.Sdk only work with net462.
Is there a reason why these packages are referenced?

[mxschmitt]

How about targeting netstandard instead?

I didn't do it because of:

Playwright.Xunit.csproj : warning NU1701: Package 'xunit.runner.visualstudio 2.8.0' was restored using '.NETFramework,Version=v4.6.1, .NETFramework,Version=v4.6.2, .NETFramework,Version=v4.7, .NETFramework,Version=v4.7.1, .NETFramework,Version=v4.7.2, .NETFramework,Version=v4.8, .NETFramework,Version=v4.8.1' instead of the project target framework '.NETStandard,Version=v2.0'. This package may not be fully compatible with your project.

Since removing #3063 (comment) sounds good to me we can target netstandard2.0 now.

xunit.runner.visualstudio and Microsoft.NET.Test.Sdk only work with net462. Is there a reason why these packages are referenced?

Addressed.

Thanks both!