selftests/clone3: flush stdout and stderr before clone3() and _exit()

Buffers have to be flushed before clone3() to avoid double messages in
the log.

Fixes: 41585bbe ("selftests: add tests for clone3() with *set_tid")
Signed-off-by: Andrei Vagin <avagin@gmail.com>
Link: https://lore.kernel.org/r/20191118064750.408003-1-avagin@gmail.com


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