tests/mem_alloc: check that malloc() objects can be accessed

Adds tests checking that we can use memory allocated by malloc(),
calloc() and realloc() to access objects of some common types,
including uint64_t and double.

It works by doing a number of allocations of various sizes and
dereferencing the returned pointer. The purpose is to catch cases where
the application would trap if accessing the allocated memory. (The
test does not check alignment against the ABI or alignof().)

Signed-off-by: Martin Åberg <martin.aberg@gaisler.com>