tests: lib: mem_alloc: Fix incorrect newlib variant usage

NEWLIB_LIBC_NANO defaults to y when building with a toolchain that
supports nano.specs and this was causing the libraries.libc.newlib
test to link with the newlib nano variant (libc_nano.a) when it should
be linking with the normal newlib (libc.a).

By setting CONFIG_NEWLIB_LIBC_NANO=n in prj_newlib.conf, we make sure
that the libraries.libc.newlib test links with the normal newlib.

For more details, refer to the issue #21167.

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