Tests: Add and fix tests for Backend/Base/*:

[emibcn]

• Backend/Base/Cache:
  • Tests: abstract and move fetch mocking code and AbortError definition to testHelpers and use it
  • Code: Use Backend/Base/Common as a base class of FetchCacheElement
• Backend/Base/Common:
  • Tests: Add tests
  • Code:
    • Fix bug in handleFetchErrors (throw Error instead of return it)
    • Abstract a bit to allow using it as base class in FetchCacheElement
• Backend/Base/GHPages:
  • Tests: Add tests
  • Code:
    • Move conditional log logic to log function
    • Return some async functions return value and await to some function calls to allow await on it and simplify tests
• testHelpers: Transform message console log catchers to wrap async functions (works well for non-async functions, too)
• ErrorCatcher: Use message console log catchers as async functions