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/Microsoft.Extensions.Testing.Abstractions.UnitTests/project.json

@@ -21,10 +21,8 @@
   "frameworks": {
     "netcoreapp1.0": {
       "imports": [
-        "netstandardapp1.5",
-        "dnxcore50",
-        "netstandard1.3",
-        "portable-net45+win8"
+        "dotnet5.4",
+        "portable-net451+win8"
       ]
     }
   },