selftests/clone3: check that all pids are released on error paths

This is a regression test case for an issue when pids have not been
released on error paths.

Signed-off-by: Andrei Vagin <avagin@gmail.com>
Link: https://lore.kernel.org/r/20191118064750.408003-3-avagin@gmail.com


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