Enable tests for task-05-01 that creates temporary files

Fix bug and enable tests from file [test-05-11.cpp](https://github.com/c71n93/cpp-course/blob/c797ab61abbb99219f30cf1ab6aa1cb56ba5430e/week-8/task-05-11/test/test-05-11.cpp) that are disabled via `TURN_OFF_TEMPFILE_TESTS` now.