Fix installation failures for versions 0.10.48, 0.12.18 and 5.12.0.

npm install for these versions is failing due to a bug that causes
installation of native modules to fail with the error:
C:\Program Files (x86)\nodejs\node_modules\npm\node_modules\node-gyp\src\win_delay_load_hook.c(34):
error C2373: '__pfnDliNotifyHook2': redefinition; different type modifiers
[C:\projects\tedious\node_modules\sspi-client\build\sspi-client.vcxproj]

This seems to be a known issue with certain npm/node-gyp versions:
nodejs/node-gyp#972
nodejs/node#7286

Fixing this by installing the latest version of npm before running the
tests.

appveyor.yml

@@ -37,6 +37,8 @@ test_script:
   - cmd: |
       SET EXITVAL=0
 
+      npm install -g npm
+
       npm run-script test || SET EXITVAL=1
 
       powershell %cd%\scripts\appveyor\config.ps1 SQL2016