selftests/nsfs: create kconfig fragments

Create a config fragment for nsfs to enable additional config options.
The config fragments can be used with the help of
scripts/kconfig/merge_config.sh.
Signed-off-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Acked-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/nsfs/config

+CONFIG_USER_NS=y
+CONFIG_UTS_NS=y
+CONFIG_PID_NS=y