fiber: mark stack slab leak on mprotect fail as a non-leak

With new ASAN-friendly small implementation unit/fiber_stack.c test
start to fail. The issue is leak sanitizer reports a leak. This is an
expected leak of test for mprotect failure on fiber stack destruction.
Let's tell sanitizer to ignore this case.

By the way let's drop test code for temporary redirecting stderr. It is
outdated as test is TAP-compatible. It was a PITA as due to this
redirection there was no leak report only error exit code.

Part of #7327

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