Use newer npm on AppVeyor (as a workaround for https://github.com/dotnet/cli/issues/6561 and dotnet/msbuild#406), and run correct test command

Author: Daniel15

appveyor.yml

@@ -2,10 +2,12 @@ version: '{build}'
 os: Visual Studio 2017
 install:
 - set PATH=%ProgramFiles(x86)%\Microsoft Visual Studio\2017\Community\MSBuild\15.0\Bin\;%PATH%
+- ps: Install-Product node 6
+- npm install --global npm
 build:
   project: build.proj
   verbosity: normal
 test_script:
-- dotnet test --configuration Release --no-build tests/React.Tests
+- dotnet test --configuration Release --no-build tests/React.Tests/React.Tests.csproj
 artifacts:
 - path: output\*.nupkg