Remove implicit references for non-test projects (#1037)

src/Logging/Logging.Testing/src/Microsoft.Extensions.Logging.Testing.csproj

@@ -5,6 +5,8 @@
     <TargetFrameworks>netstandard2.0;net461</TargetFrameworks>
     <NoWarn>$(NoWarn);CS1591</NoWarn>
     <PackageTags>$(PackageTags);testing</PackageTags>
+    <!-- This is actually a library for test projects, not a test project. -->
+    <IsUnitTestProject>false</IsUnitTestProject>
     <!-- This is an internal package for testing purposes only. -->
     <IsPackable>true</IsPackable>
     <IsShipping>false</IsShipping>
@@ -21,6 +23,15 @@
     <Reference Include="Microsoft.Extensions.Logging" />
     <Reference Include="Serilog.Extensions.Logging" />
     <Reference Include="Serilog.Sinks.File" />
+
+    <!--
+      This intentionally does not reference 'xunit', 'xunit.core', or any runner packages.
+      XUnit recommends only using xunit.extensibility.*, xunit.assert, and xunit.abstractions for packages which "extend" xunit.
+      This allows consumers to decide which type of xunit runner they want to use to run these tests,
+      and avoids problems with `dotnet pack`.
+
+      See https://xunit.github.io/docs/nuget-packages and the special note in https://xunit.github.io/releases/2.3.
+    -->
     <Reference Include="xunit.abstractions" />
     <Reference Include="xunit.assert" />
     <Reference Include="xunit.extensibility.execution" />