Feature/231 focus async

[egil]

Closes #231

PR meta checklist

• Pull request is targeting at DEV branch.
• Pull request is linked to all related issues, if any.
• I have read the CONTRIBUTING.md document.

Content checklist

• My code follows the code style of this project and AspNetCore coding guidelines.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• I have updated the appropriate sub section in the CHANGELOG.md.
• I have added, updated or removed tests to according to my changes.
  • All tests passed.

[codecov]

Codecov Report

Merging #260 (0292a9a) into dev (d4ede22) will increase coverage by 0.13%.
The diff coverage is 91.93%.

@@            Coverage Diff             @@
##              dev     #260      +/-   ##
==========================================
+ Coverage   80.39%   80.53%   +0.13%     
==========================================
  Files         110      116       +6     
  Lines        3458     3508      +50     
  Branches      443      444       +1     
==========================================
+ Hits         2780     2825      +45     
- Misses        543      547       +4     
- Partials      135      136       +1     

Impacted Files	Coverage Δ
src/bunit.core/Rendering/RootRenderTree.cs	100.00% <ø> (ø)
...bunit.core/Rendering/RootRenderTreeRegistration.cs	100.00% <ø> (ø)
...c/bunit.web/JSInterop/JSRuntimeAssertExtensions.cs	92.10% <ø> (+5.26%)	⬆️
...ubles/Authorization/FakeAuthorizationExtensions.cs	100.00% <ø> (ø)
src/bunit.core/Rendering/TestRenderer.cs	94.02% <28.57%> (-3.63%)	⬇️
...it.web/JSInterop/JSRuntimeInvocationHandlerBase.cs	84.84% <84.84%> (ø)
src/bunit.core/RazorTesting/FragmentContainer.cs	100.00% <100.00%> (ø)
src/bunit.core/TestContextBase.cs	100.00% <100.00%> (ø)
...it.web/Extensions/TestServiceProviderExtensions.cs	100.00% <100.00%> (ø)
src/bunit.web/JSInterop/BunitJSInterop.cs	100.00% <100.00%> (ø)
... and 16 more

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