tests pipe_api: Fix unitialized variable warning

Fix so the compiler does not throw a warning thinking that those
buffers may be used unitialized inside k_pipe_write/read().

Note clang does not the array size being of a variable size and
initialized as it throws a
"error: variable length array folded to constant array as an extension"

The issue was discovered with with gcc 14.2

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>