pipes: Add test that pipe data is restored without errors
pipes: Add test that pipe data is restored without errors

This eliminates the warning below as well

 | pipes.c: In function ‘open_pipe’:
 | pipes.c:252:6: error: variable ‘ret’ set but not used [-Werror=unused-but-set-variable]

Signed-off-by: Cyrill Gorcunov <7a1ea01eee6961eb1e372e3508c2670446d086f4@openvz.org>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@parallels.com>

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