Add unit tests for FontVariation

[zaveshaa]

What problem(s) does this PR solve?

Adds unit tests for FontVariation, which currently has none (its sibling FontFile already has tests).

New file tests/scene/test_fontvariation.cpp:

• default values
• setters/getters round-trips, including negative values stored as-is
• dictionary copy semantics for set_variation_opentype() / set_opentype_features()
• extra spacing setters/getters and out-of-bounds type rejection
• base font assignment, nested variations, reset to null

Additional information

• No SCsub changes needed, tests/ uses a recursive glob.
• I couldn't compile locally (Apple Clang 14, master requires 16), so CI confirmation is appreciated.
• AI disclosure: an AI assistant helped me check API usage against current sources; test scenarios are my own.

[AThousandShips]

and to draft documentation of the changes;

Please do not use AI to write PR descriptions etc.

[zaveshaa]

Understood, sorry about that — I rewrote the description in plain form and will keep descriptions concise going forward.