Thumbnail test: re-encode a small image so it cannot time out (closes #153)

The test "re-encodes smaller until the thumbnail fits the recorded size"
built a noisy 400x300 JPEG and encoded it several times, which took close
to the 5 s test limit and timed out on a busy host. It now uses a noisy
64x48 JPEG. The recorded size is still one byte less than the default
quality-50 encoding, so the first encoding still cannot fit and the test
still proves that quak re-encodes smaller until it fits. Every assertion
is unchanged.

Model: opus-5-5

test/thumbnails/thumbnails.test.ts

@@ -787,12 +787,13 @@ describe("fixMissingThumbnails", () => {
     });
 
     it("re-encodes smaller until the thumbnail fits the recorded size", async () => {
-        // A noisy 400x300 JPEG, which the default encoding (quality 50, not
+        // A noisy 64x48 JPEG, which the default encoding (quality 50, not
         // resized because it is under 720 px) cannot compress below the size
-        // recorded here: one byte less than that encoding's ciphertext.
+        // recorded here: one byte less than that encoding's ciphertext. It is
+        // small so that each encode is quick even on a busy host.
         const fixMock = await buildThumbMock();
-        const w = 400;
+        const w = 64;
-        const h = 300;
+        const h = 48;
         const noisy = new Uint8Array(
             jpegJs.encode(
                 {