TestContext.Render(renderFragment) and MarkupMatches(renderFragment)

[egil]

This PR adds support for cleaner razor based tests by making it possible for TestContext type to render RenderFragment's. This allows us to pass RenderFragments created inline in .razor files directly to the TestContext.

Also added support for passing RenderFragments to MarkupMatches' expected argument. This looks like this in practice, e.g. in a CounterTest.razor file:

@inherits TestContext
@code
{
    [Fact]
    public void Test()
    {
        var cut = Render(@<Counter />);
        cut.Find("button").Click();
        cut.Find("p").MarkupMatches(@<p>Current count: 1</p>);
    }
}

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 #262 (6bd4872) into dev (a3fc598) will increase coverage by 1.85%.
The diff coverage is 84.26%.

@@            Coverage Diff             @@
##              dev     #262      +/-   ##
==========================================
+ Coverage   80.58%   82.44%   +1.85%     
==========================================
  Files         116      117       +1     
  Lines        3508     3566      +58     
  Branches      444      467      +23     
==========================================
+ Hits         2827     2940     +113     
+ Misses        546      479      -67     
- Partials      135      147      +12     

Impacted Files	Coverage Δ
src/bunit.core/RazorTesting/FragmentContainer.cs	100.00% <ø> (ø)
src/bunit.core/TestContextBase.cs	100.00% <ø> (ø)
src/bunit.core/TestServiceProvider.cs	96.36% <ø> (+1.72%)	⬆️
...it.web/Extensions/Internal/AngleSharpExtensions.cs	66.66% <66.66%> (+16.66%)	⬆️
...nit.web/Asserting/MarkupMatchesAssertExtensions.cs	62.41% <79.24%> (+41.95%)	⬆️
src/bunit.web/TestContext.cs	93.75% <83.33%> (-6.25%)	⬇️
src/bunit.web/Extensions/TestContextExtensions.cs	100.00% <100.00%> (ø)
...it.web/Extensions/TestServiceProviderExtensions.cs	100.00% <100.00%> (ø)
src/bunit.web/RazorTesting/Fixture.cs	94.20% <100.00%> (-0.25%)	⬇️
src/bunit.web/RazorTesting/SnapshotTest.cs	78.57% <100.00%> (-1.43%)	⬇️
... and 7 more

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