Use shared location for NUnitTestAdapter

Because of cake-build/cake#3038 the tools
folder isn't being accessed correctly, but this can use the version
of the NuGet package installed by the test projects themselves.

Build/Cake/unit-tests.cs

@@ -30,7 +30,7 @@ public override void Run(Context context)
                     Logger = "trx",
                     Parallel = true,
                     EnableCodeCoverage = true,
-                    TestAdapterPath = @"tools\NUnitTestAdapter.2.3.0\build"
+                    TestAdapterPath = @"Packages\NUnitTestAdapter.2.3.0\build"
                 }));
     }