Update dependency Shouldly to v4 (#86)

Co-authored-by: Renovate Bot <[email protected]>

Author: renovate[bot]

Roslyn.Testing/Roslyn.Testing.csproj

@@ -8,7 +8,7 @@
     <ItemGroup>
       <PackageReference Include="JetBrains.Annotations" Version="2020.1.0" />
       <PackageReference Include="Microsoft.CodeAnalysis.CSharp.Workspaces" Version="3.7.0" />
-      <PackageReference Include="Shouldly" Version="3.0.2" />
+      <PackageReference Include="Shouldly" Version="4.0.0" />
     </ItemGroup>
 
 </Project>