selftests: add tests for clone3() with *set_tid

This tests clone3() with *set_tid to see if all desired PIDs are working
as expected. The tests are trying multiple invalid input parameters as
well as creating processes while specifying a certain PID in multiple
PID namespaces at the same time.

Additionally this moves common clone3() test code into clone3_selftests.h.

Signed-off-by: Adrian Reber <areber@redhat.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/r/20191115123621.142252-2-areber@redhat.com


Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>