[Tests] appveyor use npm 6.10.3 to work-around issues w/ nyc@<15

Author: raphinesse

appveyor.yml

@@ -22,14 +22,10 @@ install:
   # Get the latest stable version of Node.js or io.js
   - ps: Install-Product node $env:nodejs_version
 
+  # Use [email protected] to work-around issues w/ nyc@<15 on Windows
+  - npm install -g [email protected]
+
   # install modules
-  - ps: >-
-      if ($env:nodejs_version -eq "4") {
-        npm install -g npm@3;
-      }
-      if ($env:nodejs_version -eq "12" -or $env:nodejs_version -eq "10") {
-        npm install -g [email protected];
-      }
   - npm install
 
   # fix symlinks