[Icons] Add an icon asset ratchet test (#7596)

* [Icons] Add an icon asset ratchet test

The icon rules were enforced by review only, so the next Inkscape export could
reintroduce a baked fill or a physical size without anyone noticing.

Add a test that rejects non-24 viewBoxes, physical sizes, editor metadata,
hardcoded paints, missing tokens, color-coded filenames, and rasters. Keep the
legacy baseline empty so the ratchet can only tighten.

* [Tests] Drop the empty icon baseline

The legacy icon set is fully paid down, so the ratchet had nothing left to
ratchet: BaselineIsWellFormed required the baseline to be empty, which made
the baseline file, the stale-entry diff and the entry-format regex
unreachable. The rules are enforced directly now.

- AssetsFollowTheRules fails on any violation of collectViolations().
- tests/icon_assets/icon-baseline.txt is gone with its reader.

---------

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

tests/icon_assets/CMakeLists.txt

@@ -0,0 +1,3 @@
+cockatrice_add_test(
+  NAME icon_assets_test SOURCES icon_assets_test.cpp DEFINITIONS COCKATRICE_SOURCE_DIR="${CMAKE_SOURCE_DIR}"
+)