tests: mem_alloc: Fix build issue with malloc test

Since malloc takes a size_t, we should use 0x7fffffff as the max size
(ie what the larget unsigned int) would normall be.  Newer newlib's have
a check that will fail building since 0xf0000000 exceeds the size.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>