refactor(font): move test fixtures into a dedicated testing segment

Mock data lived in lib/ and was re-exported through the slice's
production index.ts, advertising fixtures as part of the public API.
Move it to a testing/ segment and drop it from index.ts; consumers now
import explicitly via the $entities/Font/testing subpath.

Repoints the four mock consumers (FontApplicator story, sizeResolver
and fontCatalogStore specs, comparisonStore test) to the new subpath.

src/widgets/ComparisonView/model/stores/comparisonStore.test.ts

@@ -10,7 +10,7 @@
  */
 
 import type { UnifiedFont } from '$entities/Font';
-import { UNIFIED_FONTS } from '$entities/Font/lib/mocks';
+import { UNIFIED_FONTS } from '$entities/Font/testing';
 import { queryClient } from '$shared/api/queryClient';
 import {
     beforeEach,