selftests: Add tests for creating pidns init via setns

First testcase "pidns_init_via_setns" checks that a process can become
Pid 1 (init) in a new Pid namespace created via unshare() and joined via
setns().

Second testcase "pidns_init_via_setns_set_tid" checks that during this
process we can use clone3() + set_tid and set the pid in both the new
and old pid namespaces (owned by different user namespaces). This test
requires root to run to avoid complex setup for wrapper userns.

Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>

--
pidns_init_via_setns. Make pidns_init_via_setns_set_tid require root.

Link: https://patch.msgid.link/20260318122157.280595-5-ptikhomirov@virtuozzo.com
v6: Move wrapper userns creation for unprivileged case to the top of
Signed-off-by: Christian Brauner <brauner@kernel.org>

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