Adjust Unit test commands due to recent dotnet changes (#1686)

CONTRIBUTING.md

@@ -19,7 +19,7 @@ Building, testing, and packing use all the standard dotnet commands:
 
     dotnet restore
     dotnet build --no-restore
-    dotnet pack
+    dotnet pack -c Debug
     dotnet test --no-build /p:CollectCoverage=true /p:Include=\"[coverlet.collector]*,[coverlet.core]*,[coverlet.msbuild.tasks]*\" /p:Exclude=\"[coverlet.core.tests.samples.netstandard]*,[coverlet.tests.xunit.extensions]*\"
 
 NB. You need to `pack` before testing because we have some integration testing that consume packages