crt: tests: t_assert: write child process' output to stdout

After the child process terminates, read the rest of the data it has written
to the pipe and write it to stdout; this allows to examine assertion message
printed by the child process.

Signed-off-by: Kirill Makurin <maiddaisuki@outlook.com>
Signed-off-by: LIU Hao <lh_mouse@126.com>