stdlib: Handle various corner cases in the fallback heapsort for qsort

The previous implementation did not consistently apply the rule that
the child nodes of node K are at 2 * K + 1 and 2 * K + 2, or
that the parent node is at (K - 1) / 2.

Add an internal test that targets the heapsort implementation
directly.

Reported-by: Stepan Golosunov <stepan@golosunov.pp.ru>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>