selftests: improve vm.memfd_noexec sysctl tests

This adds proper tests for the nesting functionality of vm.memfd_noexec as
well as some minor cleanups to spawn_*_thread().

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Aleksa Sarai <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Daniel Verkamp <[email protected]>
Cc: Dominique Martinet <[email protected]>
Cc: Jeff Xu <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Shuah Khan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: cyphar