libct/int/TestExecInTTY: fix error reporting
libct/int/TestExecInTTY: fix error reporting

As buf is instantiated outside the loop, it is appended to,
so if/once an error happens, it contains the output of all previous
iterations. Not a big problem but looks a bit untidy.

Move the declaration to inside the loop.

Fixes: 06a684d6a79371df5e
Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

dqminh/runc,estesp/runc,opencontainers/runc,estesp/runc,rhatdan/runc,estesp/runc,opencontainers/runc,dqminh/runc,opencontainers/runc,kolyshkin/runc,rhatdan/runc,kolyshkin/runc,dqminh/runc,rhatdan/runc,opencontainers/runc,kolyshkin/runc,kolyshkin/runc,dqminh/runc,rhatdan/runc