test: fix one type in volatile_state test

It fixes the following compilation error:

In file included from /libpmemobj-cpp/tests/volatile_state/\
volatile_state.cpp:33:0:
/libpmemobj-cpp/tests/volatile_state/volatile_state.cpp:\
In function 'void test_vector_of_elements(pmem::obj::pool<root>&)':
/libpmemobj-cpp/tests/volatile_state/volatile_state.cpp:348:30:\
error: conversion to 'size_t {aka long unsigned int}' \
from 'std::__atomic_base<int>::__int_type {aka int}' \
may change the sign of the result [-Werror=sign-conversion]
  UT_ASSERT(v2_initialized == NUM_ELEMENTS);
                              ^