chore(deps): update dependency reactiveui.testing to v20 (#87)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

src/ReactiveMvvm.Tests/ReactiveMvvm.Tests.csproj

@@ -13,7 +13,7 @@
     <PackageReference Include="FluentAssertions" Version="6.12.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.9.0" />
     <PackageReference Include="NSubstitute" Version="5.1.0" />
-    <PackageReference Include="ReactiveUI.Testing" Version="19.6.12" />
+    <PackageReference Include="ReactiveUI.Testing" Version="20.0.1" />
     <PackageReference Include="xunit" Version="2.8.0" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.8.0">
       <PrivateAssets>all</PrivateAssets>