tests: lib: mem_alloc: Fix up conditions for "no_mem" testcases

These should run for minlibc configured with
CONFIG_MINIMAL_LIBC_MALLOC_ARENA_SIZE=0. In all other cases normal
testsuite should run.

How preprocessor #if's were put previously, some configurations,
e.g. prj_newlib.conf, had the testsuite completely skipped.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>