Updated nuget packages:

- nunit 3.11.0 -> 3.12.0
- Microsoft.NET.Test.Sdk 16.0.1 -> 16.2.0
- OpenCover 4.7.922 added

Author: kurtanr

SimpleSudokuSolver.Tests/SimpleSudokuSolver.Tests.csproj

@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
     <TargetFramework>netcoreapp2.1</TargetFramework>
@@ -9,9 +9,10 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="nunit" Version="3.11.0" />
+    <PackageReference Include="nunit" Version="3.12.0" />
     <PackageReference Include="NUnit3TestAdapter" Version="3.13.0" />
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.0.1" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.2.0" />
+    <PackageReference Include="OpenCover" Version="4.7.922" />
   </ItemGroup>
 
   <ItemGroup>