Grader: Replace `threads` task `shared-heap` with thread-safe task

[Blazefrost]

Grader: Replace `threads` task `shared-heap` with thread-safe task

[ckirsch]

@Blazefrost True. Stack separation should be covered. How about testing the shared data segment as well? If you like you could also implement Dekker's algorithm to see if that works (no atomics required).

[Blazefrost]

After second thought, I think it should be fine. The main thread creates a second one, but immediately joins with it:

selfie/grader/assignments/threads/shared-heap.c

Lines 17 to 19 in 9b97426

	if (tid)
	pthread_join(status);
	else {

Only the second thread will call malloc, assign a value to the allocated memory, and exit:

selfie/grader/assignments/threads/shared-heap.c

Lines 19 to 23 in 9b97426

	else {
	heap_variable = malloc(8);
	*heap_variable = 42;
	pthread_exit(0);

However, I'm open for recommendations of new tasks. I thought about creating a test whether the stacks are properly separated using some conditional nested function calls, depending on the thread ID, but the procedure prologue of the syscalls probably has this case covered already, otherwise the code to check would have a problem with the return address.

[Blazefrost]

There's already the shared-data.c check for the shared data section.

I've now implemented a check that sums up all integers from 1 to 20 and used Dekker's algorithm for the critical section. This additional check is able to check shared data/heap sections (due to the sum variable and Dekker's algorithm control variables) as well as distinct stacks (due to calls to enter/leave critical sections) in PR #247.

[ckirsch]

@Blazefrost Awesome! Thanks a lot.