selftests/pidfd: Fix wrong expectation

Replace a wrong EXPECT_GT(self->child_pid_exited, 0) with EXPECT_GE(),
which will be actually tested on the parent and child sides with a
following commit.

Cc: Shuah Khan <skhan@linuxfoundation.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Link: https://lore.kernel.org/r/20240511171445.904356-8-mic@digikod.net
Signed-off-by: Mickaël Salaün <mic@digikod.net>