Add tests for external album cover processing

This implements basic tests for functionality related to loading and
processing external album covers, both on the scanning size, and on the
display side.

engine/cover_test.go

@@ -19,7 +19,7 @@ var _ = Describe("Cover", func() {
 
 	BeforeEach(func() {
 		ds = &persistence.MockDataStore{MockedTranscoding: &mockTranscodingRepository{}}
-		ds.Album(ctx).(*persistence.MockAlbum).SetData(`[{"id": "222", "coverArtId": "123"}, {"id": "333", "coverArtId": ""}]`)
+		ds.Album(ctx).(*persistence.MockAlbum).SetData(`[{"id": "222", "coverArtId": "123"}, {"id": "333", "coverArtId": ""}, {"id": "444", "coverArtId": "444", "coverArtPath": "tests/fixtures/cover.jpg"}]`)
 		ds.MediaFile(ctx).(*persistence.MockMediaFile).SetData(`[{"id": "123", "path": "tests/fixtures/test.mp3", "hasCoverArt": true, "updatedAt":"2020-04-02T21:29:31.6377Z"}]`)
 	})
 
@@ -58,11 +58,21 @@ var _ = Describe("Cover", func() {
 			Expect(format).To(Equal("png"))
 		})
 
-		It("returns the default cover if album is not found", func() {
+		It("returns the external cover for the album", func() {
 			buf := new(bytes.Buffer)
 
 			Expect(cover.Get(ctx, "444", 0, buf)).To(BeNil())
 
+			_, format, err := image.Decode(bytes.NewReader(buf.Bytes()))
+			Expect(err).To(BeNil())
+			Expect(format).To(Equal("jpeg"))
+		})
+
+		It("returns the default cover if album is not found", func() {
+			buf := new(bytes.Buffer)
+
+			Expect(cover.Get(ctx, "0101", 0, buf)).To(BeNil())
+
 			_, format, err := image.Decode(bytes.NewReader(buf.Bytes()))
 			Expect(err).To(BeNil())
 			Expect(format).To(Equal("png"))