Added NUnit3TestAdapter nuget so it can work without VS extension

src/NzbDrone.Test.Common/Sonarr.Test.Common.csproj

@@ -9,6 +9,7 @@
     <PackageReference Include="Moq" Version="4.12.0" />
     <PackageReference Include="NLog" Version="4.6.6" />
     <PackageReference Include="NUnit" Version="3.12.0" />
+    <PackageReference Include="NUnit3TestAdapter" Version="3.15.1" />
     <PackageReference Include="RestSharp" Version="106.6.10" />
     <PackageReference Include="Unity" Version="5.11.1" />
   </ItemGroup>