Updated MS Tests NuGet packages to last stable versions (downgrade from the current preview versions).

Author: José Miguel Sánchez Fernández

DIV2.Format.Exporter.Tests/DIV2.Format.Exporter.Tests.csproj

@@ -7,9 +7,9 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.9.0-preview-20210127-04" />
-    <PackageReference Include="MSTest.TestAdapter" Version="2.2.0-preview-20210115-03" />
-    <PackageReference Include="MSTest.TestFramework" Version="2.2.0-preview-20210115-03" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.8.3" />
+    <PackageReference Include="MSTest.TestAdapter" Version="2.1.2" />
+    <PackageReference Include="MSTest.TestFramework" Version="2.1.2" />
   </ItemGroup>
 
   <ItemGroup>