fix(tests): fix race condition in CacheWarmer pre-cache size test

The test was checking that the buffer was drained before asserting on
cached sizes, but the buffer is cleared before processBatch completes.
Use Eventually on getCachedSizes() directly to properly wait for the
artwork caching to finish.

core/artwork/cache_warmer_test.go

@@ -180,14 +180,9 @@ var _ = Describe("CacheWarmer", func() {
 			cw := NewCacheWarmer(aw, fc).(*cacheWarmer)
 			cw.PreCache(model.MustParseArtworkID("al-1"))
 
-			Eventually(func() int {
-				cw.mutex.Lock()
-				defer cw.mutex.Unlock()
-				return len(cw.buffer)
-			}).Should(Equal(0))
-
-			sizes := aw.getCachedSizes()
-			Expect(sizes).To(ContainElements(consts.UICoverArtSize, consts.UIThumbnailSize))
+			Eventually(func() []int {
+				return aw.getCachedSizes()
+			}).Should(ContainElements(consts.UICoverArtSize, consts.UIThumbnailSize))
 		})
 	})
 })