[NO SQUASH] Meshgen tests

[numberZero]

Add compact, short information about your PR for easier understanding:

• Goal of the PR
  • Meshgen testability.
• How does the PR work?
  1. Decouple meshgen from Client, to allow testing
  2. Add mesh comparison functions
  3. Add some basic meshgen tests using those
• Does it resolve any reported issue?
  • No.
• Does this relate to a goal in the roadmap?
  • Yes, 2.2 Internal code refactoring
• If not a bug fix, why is this PR needed? What usecases does it solve?

To do

This PR is Ready for Review

How to test

• Run Minetest (any game), make sure the nodes are still rendered
• Run unit tests

[numberZero]

Take a look at non-test code changes here, please. Some changes are necessary for testability (since full Client is not mockable at all) but there may be other ways.

[sfan5]

lgtm

[Zughy]

@numberZero rebase needed

[numberZero]

@numberZero rebase needed

For a draft?

[Zughy]

I thought it was kind of ready due to the sfan5's "lgtm", my bad

[numberZero]

Ready for review.

There are only a few tests included; more should follow in other PRs. E.g. for facedir, raillike, connected nodeboxes...

[SmallJoker]

Fine by me. The added unittests pass.

Added a short description for test_mesh_compare.cpp to understand what it does and squashed the commit into "Add mesh comparison functions, for tests".