Fix test project imports to be consistent.

- Every project.json needs portable-net451+win8 and dotnet5.4 imports (required by dotnet-test-xunit).
- If a test references NuGet, it also needs "netstandardapp1.5", because that the TFM NuGet uses currently.

test/TestingAbstractions/TestAppWithFullPdbs/project.json

@@ -7,11 +7,6 @@
     "NETStandard.Library": "1.5.0-rc2-24018"
   },
   "frameworks": {
-    "netstandard1.5": {
-      "imports": [
-        "portable-net45+win8",
-        "dnxcore50"
-      ]
-    }
+    "netstandard1.5": { }
   }
 }