Fix types in enumerable_thread_specific_access test

It fixes the following compilation error on openSUSE Leap:
In file included from /libpmemobj-cpp/tests/enumerable_thread_specific_access/enumerable_thread_specific_access.cpp:33:0:
/libpmemobj-cpp/tests/enumerable_thread_specific_access/enumerable_thread_specific_access.cpp: In function 'void test(pmem::obj::pool<root>&)':
/libpmemobj-cpp/tests/enumerable_thread_specific_access/enumerable_thread_specific_access.cpp:100:19: error: conversion to 'size_t {aka long unsigned int}' from 'int' may change the sign of the result [-Werror=sign-conversion]
   UT_ASSERT(n_100 + n_zeros == concurrency);
             ~~~~~~^~~
/libpmemobj-cpp/tests/common/unittest.hpp:130:11: note: in definition of macro 'UT_ASSERT'
  ((void)((cnd) ||                                                       \
           ^~~