Fix nunit tests adapter losing async locals

[maxkatz6]

What does the pull request do?

NUnit actively uses AsyncLocal to globally store TestContext.
Avalonia Headless integration with NUnit was messing up with ExecutionContext causing weird bugs like:

    [AvaloniaTest]
    public void Test1() {
        // Executes first.
    }

    [AvaloniaTest]
    public void Test2() {
        // Executes second, but test context is from the first test.
        var name = TestContext.CurrentContext.Test.Name;
        Assert.That(name, Is.EqualTo("Test2")); // Fails.
    }

How was the solution implemented (if it's not obvious)?

I couldn't understand exactly how was it broken, but my hypothesis is:

1. Avalonia headless runs a dedicated thread with its own ExecutionContext.
2. NUnit caches and saves current TestContext in the AsyncLocal (which uses ExecutionContext) - https://github.com/nunit/nunit/blob/v3.13.3/src/NUnitFramework/framework/Internal/TestExecutionContext.cs#L154
3. It picks up first TestContext of the first running test, caches it, and never invalidates.
4. NUnit itself resets TestContext via EstablishExecutionEnvironment. But it has no effect on Avalonia Tests, which don't use the same ExecutionContext.
5. Manually calling EstablishExecutionEnvironment on each tests also doesn't work, as it resets SynchronizationContext (!!!!) to NUnit's one as well, breaking anything Dispatcher related.

And it was fixed by explicitly capturing ExecutionContext of the invoking test framework (NUnit in this case), and explicitly running tests in the same context, while keeping our own SynchronizationContext.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

Obsoletions / Deprecations

Fixed issues

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0049593-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0049613-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]