[tests] Switch msbuild-mac to use package references. (dotnet#9245)

And don't specify the solution path anymore, because we can restore using just
the csproj just fine now.

Author: rolfbjarne

tests/msbuild-mac/msbuild-mac.csproj

@@ -33,9 +33,8 @@
     <Reference Include="System" />
     <Reference Include="System.Core" />
     <Reference Include="System.Xml" />
-    <Reference Include="nunit.framework">
-      <HintPath>..\..\packages\NUnit.3.11.0\lib\net40\nunit.framework.dll</HintPath>
-    </Reference>
+    <PackageReference Include="NUnit" Version="3.12.0" />
+    <PackageReference Include="NUnit.ConsoleRunner" Version="3.11.1" />
   </ItemGroup>
   <ItemGroup>
     <Folder Include="Resources\" />
@@ -75,9 +74,6 @@
       <Link>Cache.cs</Link>
     </Compile>
   </ItemGroup>
-  <ItemGroup>
-    <None Include="packages.config" />
-  </ItemGroup>
   <Import Project="$(MSBuildBinPath)\Microsoft.CSharp.targets" />
   <Import Project="CustomBuildActions.targets" />
 </Project>

tests/msbuild-mac/packages.config

@@ -82,7 +82,6 @@ public IEnumerator<RunTestTask> GetEnumerator ()
 					MSBuildTask build = new MSBuildTask (jenkins: jenkins, testProject: project, processManager: processManager);
 					build.Platform = platform;
 					build.CloneTestProject (jenkins.MainLog, processManager, project);
-					build.SolutionPath = project.SolutionPath;
 					build.ProjectConfiguration = config;
 					build.ProjectPlatform = project.Platform;
 					build.SpecifyPlatform = false;