Grader crashes when selfie is a directory

[ChristianMoesl]

The grader crashes, when grading a repository with a folder selfie located at the root of the repository.

User/Selfie: Traceback (most recent call last):
  File "grader/self.py", line 998, in <module>
    main(sys.argv)
  File "grader/self.py", line 992, in main
    do_bulk_grading(tests)
  File "grader/self.py", line 949, in do_bulk_grading
    check_assignments(tests)
  File "grader/self.py", line 964, in check_assignments
    test_base(mandatory=True)
  File "grader/self.py", line 699, in test_base
    test_compile_warnings('selfie.c', 'self-compilation does not lead to warnings or syntax errors', mandatory=mandatory)
  File "grader/self.py", line 465, in test_compile_warnings
    test_execution('./selfie -c {}'.format(file), msg, success_criteria=has_no_compile_warnings, mandatory=mandatory)
  File "grader/self.py", line 353, in test_execution
    returncode, output, _ = execute(command)
  File "grader/self.py", line 197, in execute
    process = Popen(command.split(' '), stdout=PIPE, stderr=PIPE)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/subprocess.py", line 775, in __init__
    restore_signals, start_new_session)
  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/subprocess.py", line 1522, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
PermissionError: [Errno 13] Permission denied: './selfie'