fixed test6 to include inner local tasks

my own implementation leaked memory because of the custom allocator
which wasn't tested before

changed Makefile to set NDEBUG for benchmarking

remove asserts. Critical failure must be aborted even when NDEBUG is on

Fix unchecked return value to prevent VSCode compiler errors

This error was reported by a student, who noticed when compiling in VSCode, the following error message was generated:

```
threadpool_lib.c: In function 'count_number_of_threads':
threadpool_lib.c:46:9 error: ignoring return value of 'fgets', declared with attribute warn_unused_result [-Werror=unused-result]
    fgets(buf, sizeof buf, p);
    ^
cc1: all warnings treated as errors
<builtin>: recipe for target 'threadpool_lib.o' failed
make: *** [threadpool_lib.o] Error 1
```

I was not able to reproduce this problem in `clang` or `gcc` on my machine, but I figured it was worth the PR. We should be checking return values anyways.

See merge request !4