Add two more tests for Swift staticweb middleware

Enhance the existing test to check the following functions:

web-listings-css: Custom CSS for file listing
web-error: Custom error page

Change-Id: Ib41d9593d1d3725336caf8ced62bde6d206174b5
Implements: blueprint test-swift-staticweb-middelware
Merge "Add two more tests for Swift staticweb middleware"