Only add coverlet to test projects

Don't add coverlet to non-test projects as it can cause warnings with dotnet test.

Author: martincostello

Directory.Packages.props

@@ -19,7 +19,7 @@
     <PackageVersion Include="xunit.runner.visualstudio" Version="2.4.3" />
   </ItemGroup>
   <ItemGroup>
-    <PackageReference Include="coverlet.msbuild" PrivateAssets="All" />
+    <PackageReference Include="coverlet.msbuild" PrivateAssets="All" Condition=" '$(IsTestProject)' == 'true' " />
     <PackageReference Include="Microsoft.SourceLink.GitHub" PrivateAssets="All" />
     <PackageReference Include="ReportGenerator" PrivateAssets="All" />
     <PackageReference Include="StyleCop.Analyzers" PrivateAssets="All" />