selftests: Refactor 'peeksiginfo' ptrace test part

peeksiginfo creates an array of 10 instances of 'siginfo_t',
but actually uses only one. This patch will reduce amount
of memory on the stack used by the peeksiginfo test.

Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>