Add coverlet lib.

Author: jafin

PdfSharpCore.Test/PdfSharpCore.Test.csproj

@@ -7,6 +7,14 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <PackageReference Include="coverlet.collector" Version="3.1.2">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
+    <PackageReference Include="coverlet.msbuild" Version="3.1.2">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
     <PackageReference Include="FluentAssertions" Version="6.1.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.11.0" />
     <PackageReference Include="xunit" Version="2.4.1" />