tests: lib: c_lib testcase running with 32KB min ram

Limit the testcase to targets with more than 32KB RAM
Because of the CONFIG_NEWLIB_LIBC_MIN_REQUIRED_HEAP_SIZE 8192,
the RAM used by the testcase on a too small target might fail
at runtime. For example on a nucleo_l073rz target:
    RAM:       15292 B        20 KB     74.67%

Signed-off-by: Francois Ramu <francois.ramu@st.com>