Entity Tree: Tests: Cover logic that calculates children of each Node type

Why:

A regression in functionality was almost released as part of Crystal v1.7.0b:
- Issue: After expanding the first group, expanding any future _ResourceNode would never show root resources as RootResourceNodes correctly, instead showing them as NormalResourceNodes
- Detected by:
  - Ad-hoc manual testing by downloading a non-trivial site with Crystal (minimalistbaker.com)
- Fixed by:
  - COMMIT: Fix preload of resources to reuse existing Resource objects if available.
  - However this commit did not include a regression test, since it would be part of the larger suite that this GH Issue is talking about
(?) Some known bugs would become increasingly obvious, which would encourage actually fixing them
- Not yet in ISSUE:
  - FTR-12: After selecting an URL in the entity tree, and using the "Add URL" button to promote it to a root URL, the entity tree updates its arrangement of entities but does NOT update the selected URL to match the one that was selected before. Instead the selection is left in a seemingly random place. This is confusing.

In particular, check things like:

For a _ResourceNode, does it calculate its children correctly?
- Links to root resources pulled to the top correctly?
- Links to group members grouped correctly?
- Links to other resources shown?
- Embedded resources grouped & hidden correctly?
- Off-site resources grouped & hidden correctly?
For a RootNode, does it calculate its children correctly?
- Root resources listed at the top, in order of creation?
- Groups listed below root resources, in order of creation?
- Nothing else below groups?
For a ResourceGroupNode, does it calculate its children correctly?
- (I think this may be covered by existing tests that expand the "X more..." link)

davidfstr / Crystal-Web-Archiver