kselftests: Add test to check for rlimit changes in different user namespaces

The testcase runs few instances of the program with RLIMIT_NPROC=1 from
user uid=60000, in different user namespaces.

Signed-off-by: Alexey Gladkov <legion@kernel.org>
Link: https://lkml.kernel.org/r/28cafdcdd4abd8494b34a27f1970b666b30de8bf.1619094428.git.legion@kernel.org
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>