tests: cpp: libcxx: Set minimum RAM requirement for full newlib test

This commit sets the minimum RAM requirement for the full newlib test
(`cpp.libcxx.newlib`) to 24 KiB so that only the target platforms that
can provide sufficient RAM area for the newlib heap are selected.

In case of the newlib full variant, the minimum required newlib heap
size, specified by CONFIG_NEWLIB_LIBC_MIN_REQUIRED_HEAP_SIZE, is 8192.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>