File System Simulator

Design and implement a simplified in-memory file system. Your file system should support the following operations:

1. ls(path): Given a path in the file system, return a list of files and directories in lexicographical order. If the path is a file path, return a list containing just the file name.

2. mkdir(path): Create a new directory at the specified path. If intermediate directories do not exist, create them as well.

3. addContentToFile(filePath, content): Create a file if it does not exist and append the given content to the file.

4. readContentFromFile(filePath): Return the content of the file at the specified path.

Assume the root path is '/' and all paths are absolute. Your design should efficiently support the operations above. Implement your solution in a programming language of your choice and include unit tests to demonstrate the correctness of your implementation.