On the current main branch, running multi-line command through sandbox:

# testing
print('test multi-line command')
exit_code, output = ssh_box.execute("pwd\nls -l\nls -l")
print('exit code:', exit_code)
print(output)

would timeout and/or have issues, since the while loop will be looking for command_output that startswith cmd.

This PR:

• remove those complex logic and fix this issue.
• add unit tests of single-line & multi-line commands to test

Will resolve #1431