tests: disable node > 12 tests on windows, nyc fails to launch

nyc needs upgrading to v15, but v15 only works in node versions that support let/const

Author: Salakar

appveyor.yml

@@ -6,8 +6,9 @@ environment:
     - nodejs_version: "4"
     - nodejs_version: "6"
     - nodejs_version: "8"
-    - nodejs_version: "12"
-    - nodejs_version: "13"
+    - nodejs_version: "10"
+#    - nodejs_version: "12"
+#    - nodejs_version: "13"
 
 pull_requests:
   do_not_increment_build_number: true