Also fix UTF-8 inner byte bounds and make unittest for case this fixes.

Also count multibyte sequences as "one" character.
Adds unittest for the bug reporter's case.
Fixes #2796.

Improve schematic file-saving interface
Add ability to create temporary test files

This simplifies NodeResolver logic and makes some interfaces cleaner.

- Add ability to explicitly reset NodeResolve state (useful for unittesting)
- Remove non-essential NodeResolve methods modifying state from INodeDefManager
- Add const qualifier to NodeDefManager and ContentFeatures serialize

Minor misc. NodeResolver cleanups
Prefix faux content type constants for testing with t_ to avoid
confusion or name collisions

-> Round negative numbers correctly CMakeLists.txt
-> Link Json with the static run-time library

Make sure to include random unittests in android builds, too
Use SWAP() macro
Ensure that negative ranges are tested as well in random unittests

Split unit tests into separate files under src/unittest/
Give better unittest diagnostics
Clean up some code