refactor: prune test-only seams and obsolete helpers (#10275)

tests/fixtures/hopperImageCopyFileSystem.ts

@@ -0,0 +1,24 @@
+import { open } from "node:fs/promises";
+
+/** Observe completed writes to a real prepared-image file. */
+export const hopperImageCopyFileSystem = (
+  destination: string,
+  afterWrite: (bytes: number) => Promise<void>,
+  openFile: typeof open = open,
+) => ({
+  open: (async (...args: Parameters<typeof open>) => {
+    const handle = await openFile(...args);
+    if (args[0] === destination) {
+      const writeFile = handle.writeFile.bind(handle);
+      handle.writeFile = (async (
+        ...writeArgs: Parameters<typeof writeFile>
+      ) => {
+        await writeFile(...writeArgs);
+        const data = writeArgs[0];
+        if (!Buffer.isBuffer(data)) throw new Error("Expected a buffer write");
+        await afterWrite(data.length);
+      }) as typeof handle.writeFile;
+    }
+    return handle;
+  }) as typeof open,
+});