test: fix flaky CacheWarmer deduplication test

Fixed race condition in the 'deduplicates items in buffer' test where the
background worker goroutine could process and clear the buffer before the
test could verify its contents. Added fc.SetReady(false) to keep the cache
unavailable during the test, ensuring buffered items remain in memory for
verification. This matches the pattern already used in the 'adds multiple
items to buffer' test.

Signed-off-by: Deluan <deluan@navidrome.org>

core/artwork/cache_warmer_test.go

@@ -90,6 +90,7 @@ var _ = Describe("CacheWarmer", func() {
 		})
 
 		It("deduplicates items in buffer", func() {
+			fc.SetReady(false) // Make cache unavailable so items stay in buffer
 			cw := NewCacheWarmer(aw, fc).(*cacheWarmer)
 			cw.PreCache(model.MustParseArtworkID("al-1"))
 			cw.PreCache(model.MustParseArtworkID("al-1"))