Add additional npm install step to fix package tests under node6 (#579)

Author: DmitriiBobreshev

azure-pipelines.yml

@@ -5,23 +5,32 @@ strategy:
   matrix:
     'Node 6':
       versionSpec: '6.x'
+      npmVersion: '5'
     'Node 8': 
       versionSpec: '8.x'
+      npmVersion: '6'
     'Node 10': 
       versionSpec: '10.x'
+      npmVersion: '6'
     'Node 12': 
       versionSpec: '12.x'
+      npmVersion: '6'
     'Node 14': 
       versionSpec: '14.x'
+      npmVersion: '6'
     'Node 16':
       versionSpec: '16.x'
+      npmVersion: '8'
 
 steps:
 - task: NodeTool@0
   inputs:
     versionSpec: $(versionSpec)
   displayName: Install node
 
+- script: npm install -g npm@$(npmVersion)
+  displayName: Upgrade npm
+
 # install dependencies
 - script: npm install
   displayName: npm install