selftests: lib.mk: Do not process TEST_GEN_MODS_DIR

The directory itself doesn't need have path handling, since it's only to
mean where is the directory that contains modules to be built.

Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>