[Tests] install npm 5 before npm 6 in node 7 on windows

Author: ljharb

appveyor.yml

@@ -52,6 +52,7 @@ install:
   - IF %nodejs_version% EQU 4 npm -g install npm@5.3
   - IF %nodejs_version% EQU 5 npm -g install npm@5.3
   - IF %nodejs_version% EQU 6 npm -g install npm@6.9
+  - IF %nodejs_version% EQU 7 npm -g install npm@5
   - IF %nodejs_version% EQU 7 npm -g install npm@6
   - IF %nodejs_version% EQU 8 npm -g install npm@6
   - IF %nodejs_version% EQU 9 npm -g install npm@6.9