Added handling of fixtures that implement IAsyncLifetime, IAsyncDisposable

[candoumbe]

I took the liberty of opening this PR to fix #5
I also added parallelization

Hope this will help the community

[JDCain]

Thank you for this! I failed to understand the issue the other person had posted.

I am not familiar with Parallel.ForEach() & .ForEachAsync(), why did you use it instead of something like Task.WhenAll() which is used in the XunitTestClassRunner?

[candoumbe]

The idea behind using Parallel.ForXXX was just to parallelize initialization and destruction calls.
But yeah maybe this could go a step further and, when supported parallelize all possible tasks. But I wonder how to do this as not all things are asynchronous.

Could it be done as an improvement in an other PR ?

[siewers]

I can see the usage of Parallel.ForEach was discussed here. This does not work and is currently broken in .NET 5 and below.
The alternative is to use Task.WhenAll, which in fact does work.
I haven't investigated how this works in .NET 6, where the Parallel.ForEachAsync exists, but this is not valid for earlier versions.

I can create a working PR for fixing the implementation in .NET 5 and below.