Add failing tests for the format auto

Tests for #88, before the change:
the format chosen for each Accept header (AVIF-capable, WebP-only,
neither, wildcard only, absent, q=0 on AVIF, malformed), both image
routes serving an auto URL in the chosen format with Vary: Accept, also
on HEAD and 304, the signature covering auto rather than the chosen
format, each chosen format cached apart, and Vary: Accept next to the
CORS middleware's Vary: Origin. They do not compile yet: FormatAuto and
formatForAccept do not exist.

Model: opus-5-5

internal/imgcache/urlparser_internal_test.go

@@ -95,6 +95,14 @@ func TestParseImageURL(t *testing.T) {
 				Size: Size{Width: 200, Height: 200}, Format: FormatGIF,
 			},
 		},
+		{
+			name:  "auto format",
+			input: "/v1/image/example.com/photo.jpg/200x200.auto",
+			want: &ParsedURL{
+				Host: testHostExample, Path: "/photo.jpg",
+				Size: Size{Width: 200, Height: 200}, Format: FormatAuto,
+			},
+		},
 	}
 
 	for _, tt := range tests {