test/mocks: add cockpit modify

Add the `modify` file function which performs atomic modifications to a
file.

src/test/mocks/cockpit/cockpitFile.ts

@@ -32,7 +32,7 @@ export const getLastBlueprintReq = () => {
   return lastRequest.blueprints[lastRequest.blueprints.length - 1];
 };
 
-export const cockpitFile = (filepath: string) => {
+export const cockpitFile = (filepath: string, options?: object) => {
   return {
     read: (): Promise<string> => {
       const file = path.parse(filepath);
@@ -52,5 +52,10 @@ export const cockpitFile = (filepath: string) => {
         lastRequest.blueprints.push(contents);
       }
     },
+    modify: (callback: (contents: string) => string): Promise<string> => {
+      return new Promise((resolve) => {
+        resolve(callback(''));
+      });
+    },
   };
 };