http API test support

[WeihanLi]

Testing spec

• environment
• collection(request collection)
• group(request group in a collection)
• request(specific request)
• variable

collectionName: testCollection
  variables:
    - name: value
  preScript: |
      $request.headers.add("apiKey", "test") 

  - groupName: testGroup
      variables:
        - name: value
      - requestName: hello request
        variables:
          - name: value      
        postScript: "$response.EnsureSuccessStatusCode()"

environment:
  name: dev
  variables:
    - name: value