[OpenMP] Reduce the size of heap memory required by the test `malloc_parallel.c` (#75885)

This patch reduces the size of heap memory required by the test
`malloc_parallel.c` and `malloc.c`. The original size is too large such
that `malloc` returns `nullptr` on many threads, causing illegal
memory access.

GitOrigin-RevId: 7e4c6f6cb2e17ee186c9525e59218de0b2277799
2 files changed