selftests/timens: add a test for vfork+exit

* check that a child process is in parent's time namespace after vfork.
* check that a child process is in the target namespace after exec.

Output on success:
1..4
ok 1 parent before vfork
ok 2 child after exec
ok 3 wait for child
ok 4 parent after vfork
# Totals: pass:4 fail:0 xfail:0 xpass:0 skip:0 error:0

Signed-off-by: Andrei Vagin <avagin@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20221013173154.291597-1-avagin@google.com

selftests/timens: add a test for vfork+exit

* check that a child process is in parent's time namespace after vfork.
* check that a child process is in the target namespace after exec.

Output on success:
$ ./vfork_exec
1..1
ok 1 exec

Signed-off-by: Andrei Vagin <avagin@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220613060723.197407-2-avagin@gmail.com