tests: lib: mem_alloc: restrict newlib config to more than 16K RAM

This test fails for newlib configuration,
for nucleo_f030r8 which has only 8K of RAM.
But each step of the test is successfull when executed solely
or with smaller buffer size.
Note: test is working on nucleo_f091rc which has larger RAM (32K)

Signed-off-by: Alexandre Bourdiol <alexandre.bourdiol@st.com>