Update dependency Soenneker.Tests.Unit to 2.1.400 (#425)

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

Author: renovate[bot]

test/Soenneker.Utils.AsyncSingleton.Tests/Soenneker.Utils.AsyncSingleton.Tests.csproj

@@ -16,7 +16,7 @@
             <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
             <PrivateAssets>all</PrivateAssets>
         </PackageReference>
-        <PackageReference Include="Soenneker.Tests.Unit" Version="2.1.399" />
+        <PackageReference Include="Soenneker.Tests.Unit" Version="2.1.400" />
     </ItemGroup>
 
     <ItemGroup>