perf(test): speed up config tests

src/config/backup-rotation.ts

@@ -0,0 +1,26 @@
+export const CONFIG_BACKUP_COUNT = 5;
+
+export async function rotateConfigBackups(
+  configPath: string,
+  ioFs: {
+    unlink: (path: string) => Promise<void>;
+    rename: (from: string, to: string) => Promise<void>;
+  },
+): Promise<void> {
+  if (CONFIG_BACKUP_COUNT <= 1) {
+    return;
+  }
+  const backupBase = `${configPath}.bak`;
+  const maxIndex = CONFIG_BACKUP_COUNT - 1;
+  await ioFs.unlink(`${backupBase}.${maxIndex}`).catch(() => {
+    // best-effort
+  });
+  for (let index = maxIndex - 1; index >= 1; index -= 1) {
+    await ioFs.rename(`${backupBase}.${index}`, `${backupBase}.${index + 1}`).catch(() => {
+      // best-effort
+    });
+  }
+  await ioFs.rename(backupBase, `${backupBase}.1`).catch(() => {
+    // best-effort
+  });
+}