selftests/nolibc: add testcase for pipe

Add a test case of pipe that sends and receives message in a single
process.

Suggested-by: Thomas Weißschuh <thomas@t-8ch.de>
Suggested-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/all/c5de2d13-3752-4e1b-90d9-f58cca99c702@t-8ch.de/


Signed-off-by: Yuan Tan <tanyuan@tinylab.org>
Reviewed-by: Thomas Weißschuh <linux@weissschuh.net>
[wt: fixed the "len" type to size_t to address a sign-compare warning
 with upcoming patches]
Signed-off-by: Willy Tarreau <w@1wt.eu>