Use '$(SolutionDir)\packages' so that NUnit NuGet package can be found

even if AvalonEdit is used as submodule in a larger solution.

Author: dgrunwald

ICSharpCode.AvalonEdit.Tests/ICSharpCode.AvalonEdit.Tests.csproj

@@ -64,7 +64,7 @@
       <Name>ICSharpCode.NRefactory</Name>
     </ProjectReference>
     <Reference Include="nunit.framework">
-      <HintPath>..\packages\NUnit.2.6.3\lib\nunit.framework.dll</HintPath>
+      <HintPath>$(SolutionDir)\packages\NUnit.2.6.3\lib\nunit.framework.dll</HintPath>
     </Reference>
     <Reference Include="PresentationCore">
       <RequiredTargetFramework>3.0</RequiredTargetFramework>