fix(medusa-test-utils): Fixed count serialization and incorrect function call (#5090)

- Serialized `count`  - Updated the function name called inside the mock implementation

Co-authored-by: Oli Juhl <59018053+olivermrbl@users.noreply.github.com>
Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

packages/medusa-test-utils/src/mock-repository.js

@@ -29,6 +29,7 @@ class MockRepo {
     this.findAndCount_ = findAndCount
     this.findOneWithRelations_ = findOneWithRelations
     this.insertBulk_ = insertBulk
+    this.count_ = count
 
     this.metadata = metadata ?? {
       columns: [],
@@ -108,7 +109,7 @@ class MockRepo {
   })
   count = jest.fn().mockImplementation((...args) => {
     if (this.count_) {
-      return this.count(...args)
+      return this.count_(...args)
     }
     return {}
   })